Since the Will is a command and an obligation from God, the Prophet Mohammed (PBUH & His Family) is the first to implement the commands of Allah therefore he must have written a Will.
The Will of the Prophet Mohammed (PBUH & His Family):
“Prophet Mohammed (PBUH & His Family) said to Ali (PBUH) on the night of his death, ‘O Father of Al-Hassan, bring me a pen and a paper,’ so the Messenger of Allah (PBUH & His Family) dictated his Will until he came to this position where he said: ‘O Ali, there will be twelve Imams after me and after them there will be twelve Mahdis. So you, O Ali, are the first of the twelve Imams, Allah the Exalted has named you in His heavens Ali Al-Murtada (the Content), Amirul Mo’mineen (the Prince of the Believers), Al-Siddiq Al-Akbar (the Greater Truthful), Al-Farouq Al-A’tham (the Greater Judge and Differentiator between truth and falsehood), Al-Ma’moun (the Trusted), and the Mahdi (the Guided). These names may not be attributed to other than you. O Ali, you are my vicegerent/guardian over my own family, their living and their dead, and upon my women: Whomever you kept, she shall find me tomorrow, and whomever you divorced, I am innocent of her, I will not see her and she will not see me on the Day of Resurrection. And you are my successor (Khalifa) over my nation after me. If death comes to you, hand it over to my son Al-Hassan, the righteous and benevolent. Then if death comes to him, let him hand it over to my son Al-Hussein, the martyr, the pure and murdered. If death comes to him, let him hand it over to his son, the master of worshipers, Dhul Thafanat (the one with hard skin on his knees) Ali. If death comes to him, let him hand it over to his son, Mohammed Al-Baqir (the Revealer of Knowledge). If death comes to him, let him hand it over to his son, Ja’far Al-Sadiq (the Truthful). If death comes to him, let him hand it over to his son, Musa Al-Kathim (the Patient). If death comes to him, let him hand it over to his son, Ali Al-Ridha (the Pleasing One). If death comes to him, let him hand it over to his son, Mohammed Al-Thiqa Al-Taqqi (the Trustworthy, the God-Fearing). If death comes to him, let him hand it over to his son, Ali Al-Nasih (the Advisor). If death comes to him, let him hand it over to his son, Al-Hassan Al-Fadhil (the Meritorious). If death comes to him, let him hand it over to his son, Mohammed the Safeguarded of the Family of Mohammed (PBUT). Those are the twelve Imams. Then there will be twelve Mahdis after him, so if death comes to him, let him hand it over to his son, the first of the close ones, he has three names, one like mine and my Father’s and it is Abdullah (Servant of God), Ahmed (the Praised), and the third name is Al-Mahdi (the Guided), and he is the first of the believers.’ ”
- Ghaybat Al-Tusi, Al-Shaikh Al-Tusi, Vol. 1, p. 174 - AR; Bihar Al-Anwar, Al- Allamah Al-Majlisi, Vol. 53, p. 148
https://thaqalayn.net/hadith/27/1/26/11
What is the proof that “Ahmed” mentioned in the Will is the Mahdi?
The teachings of the Ahlul-Bayt (From Them is Peace) already clarify that divine appointment can not be claimed except by its rightful owner. However, the Sunni non-working scholars concealed the truth about divine successorship and divine appointment, and about the Supremacy of Allah.
From a logical point of view, since God the Almighty appoints caliphs on earth (“Indeed, I will make upon the earth a successive authority” (Qur'an, 2:30), then this process of divine successorship must depend on a divine appointment and a Will. Therefore, it makes sense that God the Almighty protects the divine Will, otherwise, people will not be able to recognize the true caliphs of God from the false claimants.
This is why the Prophet (PBUH & His Family) described his Will as “a protection of misguidance” in both Sahih books.
Sa'id b. Jubair reported from Ibn Abbas that he said: “Thursday, and what about Thursday? Then tears began to flow until I saw them on his cheeks as if they were the strings of pearls. He (the narrator) said that Allah's Messenger said: Bring me a shoulder blade and ink-pot (or tablet and inkpot), so that I write for you a document (by following which) you would never go astray. They said: Allah's Messenger is in the state of unconsciousness.
Sahih Muslim, Muslim ibn Al-Hajjaj Al-Naysaburi, Book 25, Hadith 30
https://sunnah.com/muslim:1637b
Narrated Ibn Abbas: Thursday! And how great that Thursday was! The ailment of Allah's Messenger became worse (on Thursday) and he said, fetch me something so that I may write to you something after which you will never go astray." The people (present there) differed in this matter, and it was not right to differ before a prophet. Some said, "What is wrong with him ? (Do you think ) he is delirious (seriously ill)? Ask him ( to understand his state )." So they went to the Prophet and asked him again. The Prophet said, "Leave me, for my present state is better than what you call me for." Then he ordered them to do three things. He said, "Turn the pagans out of the Arabian Peninsula; respect and give gifts to the foreign delegations as you have seen me dealing with them." (Saeed bin Jubair, the sub-narrator said that Ibn Abbas kept quiet as rewards the third order, or he said, "I forgot it.")
Sahih Bukhari, Al-Bukhari, Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi), Hadith No. 4431
https://sunnah.com/bukhari:4431
Therefore the Will of the Prophet (PBUH & His Family) is protected from being claimed by false claimants, otherwise, it won't be a ‘protection from misguidance’. The Prophet described it as such; therefore, only the truthful companion will claim it.
If we research what the Caliphs of Allah claimed to prove they were the true caliphs we will find that they all claimed to be divinely appointed. Let’s see the following examples:
1- Jesus (PBUH) claimed to be the one that Moses (PBUH) prophesied about:
"You study the Scriptures diligently because you think that in them you have eternal life. These are the very Scriptures that testify about me, If you believed Moses, you would believe me, for he wrote about me. But since you do not believe what he wrote, how are you going to believe what I say?”
The Holy Bible, Book of John, Chapter 5, Verses 46-47
https://www.biblegateway.com/passage/?search=John%205&version=NIV
Jesus (PBUH) also claimed to be the one prophesied by Isaiah (PBUH):
He went to Nazareth, where he had been brought up, and on the Sabbath day he went into the synagogue, as was his custom. He stood up to read, and the scroll of the prophet Isaiah was handed to him. Unrolling it, he found the place where it is written: “The Spirit of the Lord is on me, because he has anointed me to proclaim good news to the poor. He has sent me to proclaim freedom for the prisoners and recovery of sight for the blind, to set the oppressed free, to proclaim the year of the Lord’s favor.” Then he rolled up the scroll, gave it back to the attendant and sat down. The eyes of everyone in the synagogue were fastened on him. He began by saying to them, “Today this scripture is fulfilled in your hearing.” - The Holy Bible, Book of Luke, Chapter 4, Verses 17-21
https://www.biblegateway.com/passage/?search=Luke+4&version=NIV
2- Prophet Mohammed (PBUH & His Family) also claimed to be the one who was prophesied to come by Jesus (PBUH):
And [mention] when Jesus, the son of Mary, said, "O Children of Israel, indeed I am the messenger of Allah to you confirming what came before me of the Torah and bringing good tidings of a messenger to come after me, whose name is Ahmed." But when he came to them with clear evidences, they said, "This is obvious magic."
If anyone can simply come and claim the divine Will, then what makes us so sure that Prophet Mohammed (PBUH & His Family) or Jesus (PBUH) were truly from God? The only guarantee that they are true, is that the divine Will is protected and can only be claimed by its companion.
We therefore proved that the Will must be protected and only its rightful claimant will claim it. Thus, when Imam Ahmed Al-Hassan (From Him is Peace) claimed to be ‘Ahmed’ mentioned in the Will of Prophet Mohammed (PBUH & His Family), he must be the truthful claimant as the Will is a protection from misguidance.
J) Additional verses to contemplate over:
And if he had made up about Us some [false] sayings, We would have seized him by the right hand; Then We would have cut from him the aorta. And there is no one of you who could prevent [Us] from him. - The Holy Qur'an, 69:44-46
All Sunni scholars interpreted this verse saying that if the Prophet (PBUH & His Family) made up lies about God, then God will punish him and cut off his life. Let’s see Tafsir Ibn Kathir and Tafsir Al-Baghawi as two examples:
Tafsir Ibn Kathir, Ibn Kathir:
The Almighty says: {And if he had made up about Us some sayings}, ie: Mohammed (PBUH & His Family) if he had been, as they claim, fabricating lies about us, so he added to or omitted from the message, or said something of his own mind and attributed it to us, and it is not like that, we would have hastened to punish him. That is why he said {We would have seized him by the right hand}. It was said: It means that we would take revenge from him with the right hand; as it is more severe in punishment, and it was said: We would have seized him with his right hand. {Then We would have cut from him the aorta} Ibn Abbas said: It is the blood artery connecting the heart, and it is the artery in which the heart is suspended. The same was said by Ikrimah, Sa'id ibn Jubayr, Al-Hakam, Qatadah, Al-Dahhak, Muslim Al-Bateen, and Abu Sakhr Hamid ibn Ziyad.
Tafsir Al-Baghawi, Al-Baghawi:
“And if he made up lies, or made up {about us}, Mohammed, {some sayings}, and he brought something of his own. {We would have seized him by the right hand}, it was said meaning metaphorically: We would have seized him and took revenge from him by the right hand, i.e. by the truth."
Even if we take their interpretation, it proves that Mohammed (PBUH and His Family) cannot claim anything from his own mind and that he was the rightful claimant after Jesus (PBUH). Otherwise, God would have cut off his life.
When Imam Ahmed Al-Hassan (From Him is Peace) came to us, he brought to us the apparent Will.
It was narrated from Abd Al A'ala that he said, I said to Abi Abdillah (From Him is Peace): 'The one who claims this matter, what is his proof?', he said: 'He is asked about the halal and haram'. He said, then Ali came and said: 'Three proofs do not gather in a person except that he is the companion of this matter: That he be the most deserving [of the right] of the one who was before him, that he has the weapon, and that he be the owner of the apparent will'.
Al-Kafi, Al-Shaikh Al-Kulayni, Vol. 1, p. 28 AR
https://thaqalayn.net/hadith/1/4/62/2 ENG
Apparent means that it is something evident amongst the people, it is known and accepted and it is not something that somebody just came up with.
And Imam Ahmed Al-Hassan (From Him is Peace) claimed to be “Ahmed” in the will of Prophet Mohammed (PBUH & His Family) and that he is the vicegerent of Imam Mahdi (From Him is Peace).
Thus it says in the Holy Qur'an: “And if he had made up concerning Us some [false] sayings. We would have seized him by the right hand. Then We would have cut from him the aorta”.
The Holy Qur'an, Chapter 69: 44-46
Imam Al-Sadiq (From Him is Peace) said, “This matter will not be claimed except by the rightful owner, or else Allah will cut off his age.”
Al-Kafi, Al-Shaikh Al-Kulayni,Vol. 1, p. 373
Bihar Al-Anwar, Al-Allamah Al-Majlisi, V25 p116
And we know that when the Mahdi appears he claims the rights to the Holy Will of Prophet Mohammed (PBUH & His Family).
From Al-Harith ibn Al-Mughira who said, ‘I said to Abu Abdullah (PBUH), ‘By what is the Companion this Matter recognised?’ He said: ‘By tranquility, and dignity, and knowledge, and the Will’’.
- Bihar Al-Anwar v25, p135
It was narrated from Abd Al A'ala that he said, I said to Abi Abdillah (PBUH) : 'The one who claims this matter, what is his proof?', he said: 'He is asked about the halal and haram'. He said, then Ali came and said: 'Three proofs do not gather in a person except that he is the companion of this matter: That he be the most deserving [of the right] of the one who was before him, that he has the weapon, and that he be the owner of the apparent will'.
Al-Kafi, Al-Shaikh Al-Kulayni, Vol. 1, p. 284
Al-Mufaddel ibn Umar had said: I heard Abu Abdullah Al-Sadiq (PBUH) saying: “The companion of this matter has two absences. After one of them he comes back to his family and regarding the other one it is said: “He has perished, which valley has he gone to?” I said: “If that takes place, then what will we do?” He said: “If a claimant claims it, you are to ask him about matters, which no one will answer except him."
Al-Kafi, Al-Shaikh Al-Kulayni, Vol.1 p. 340
Thus, the divine Will is protected by God and He the Almighty will cut off the life of anyone who claims it falsely.
Therefore, Ahmed Al-Hassan (From Him is Peace) and Abdullah Aba Al-Sadiq (From Him is Peace) both claimed the Will and are therefore the rightful companions of the Will. They are the successors whom we should hold tight to in this day and age…Peace and mercy is from them.