The Prophet Muhammad pbuhap, on the night of his death, said to Ali bin Abi Talib: “O Father of al-Hassan, bring me a pen and paper,” and he dictated his will until he came to a place where he said, “O Ali, there will be twelve Imams after me and after them there will be twelve Mahdis. You, O Ali, are the first of the twelve Imams. God has named you in His heavens Ali al-Mortada, the Prince of the Believers, the Grand Truthful, the Bright Differentiator between truth and falsehood, the trusted, and the Mahdi (the rightly-guided). These names may not to be truly attributed to other than you. O Ali, you are my successor over my family, their living and their deceased, and over my women. Whomever you affirm shall find me tomorrow, and whomever you reject I am innocent of her. I will not see her and she will not see me on the Day of Resurrection. And you are the vicegerent (khalifa) upon my nation after me. If God receives you, hand it over to my son al-Hassan, the very beneficial. Then if God receives him, let him hand it over to my son al-Hussein, the martyr, the pure, the assassinated. Then if God receives him, let him hand it over to his son, the master of the worshippers, Ali. Then if God receives him, let him hand it over to his son, Muhammed al-Baqir. Then if God receives him, let him hand it over to his son, Jafar al-Sadiq. Then if Allah receives him, let him hand it over to his son, Musa al-Khathum. Then if God receives him, let him hand it over to his son, Ali al-Retha. Then if God receives him, let him hand it over to his son Muhammad, the trustworthy, the pious. Then if God receives him, let him hand it over to his son Ali, the advisor. Then if God receives him, let him hand it over to his son al-Hassan, the virtuous. Then if God receives him, let him hand it over to his son, Muhammad, the Preserver of the Holy progeny of Muhammad pbut. These are the twelve Imams. After him there will be twelve Mahdis. So if God receives 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: Abdullah (servant of God), Ahmed, and the third name is al-Mahdi. He is the first of the believers.
Ghaybat Al Tousi (Occultation by Al Tousi) p. 96, p. 150 | Bihar Al Anwar (Oceans of Lights) v. 53, p. 148, p. 360. (via 12mahdis)