[On which madhhab will al-Mahdi (Peace be Upon Him) be]

The answer: Imam al-Mahdi (Peace be Upon Him) is from Ahl al-Bayt, from the progeny of al-Hasan or al-Husayn (Peace be Upon Them), and there is no disagreement about this.
- And the people of every madhhab expect that he will be from the people of their madhhab; so Ahl al-Sunnah expect that he will be from them, and the Ja‘fariyyah likewise, and the Isma‘iliyyah likewise, and the Zaydiyyah likewise, and ...etc.
- And naming him “al-Mahdi” is a naming that came from the Prophet (May Allah bless him and his family and grant them peace), so that indicates that Allah, the Exalted, guided him to the truth regarding that wherein the people of the Islamic madhhabs differed.
- And what came in the hadith, that he fills the earth with justice, indicates that the false madhhabs will be defeated and will end, and justice and truth will take their place.
- And upon what has preceded, al-Mahdi (Peace be Upon Him) will revive the true religion which the Messenger of Allah (May Allah bless him and his family and grant them peace) was upon, and he will cause the false madhhabs to die; and there is no disagreement among the Ummah of Muhammad (May Allah bless him and his family and grant them peace) that there will remain a group from the Ummah of Muhammad (May Allah bless him and his family and grant them peace) upon the truth; and upon that, al-Mahdi (Peace be Upon Him) will be upon the madhhab of that rightful group, not others.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3