The Prophet said about the only authentic physical description of the Mahdi. Anything else that is mentioned according to the scholars of Hadith is not authentic.
The only authentic physical description of the Mahdi is this hadith:
The Physical Sign
The Mahdi will be of my stock, and will have a broad forehead a prominent nose. He will fill the earth will equity and justice as it was filled with oppression and tyranny, and he will rule for seven years. (Authenticated by Al-Albani)
Because the concept of Al-Mahdi is so controversial and so many groups want there to be a specific type of Mahdi, there are hundreds of fabricated hadiths about Al-Mahdi.
Because, as we said, certain groups have an agenda when they want to push certain people to be the Mahdi, so in the earliest part of Islam, so many people fabricated things about Al-Mahdi.
Therefore, it is very important that we take from the scholars of Hadith that which is authentic and leave that which is not authentic.
A Spiritual Sign
Regarding his spiritual characteristics, the Prophet (peace be upon him) said that:
The Mahdi shall be from the Ahlul Bayt. Allah will rectify him in one night. (Ibn Majah)
This is an interesting hadith because it tells us that the Mahdi in the younger portion of his life will not be as practicing of a Muslim as he should be. He will not be to the level that he is worthy of being.
Something will happen we don’t know what, and in one night he shall repent and he shall become basically the most pious person on earth; he will not be born like this; he will not be raised as a scholar since the beginning memorize the Quran.
In one night Allah will purify him and correct him. So he will not be to the level of piety we would expect him to be, in one night Allah will then cause him to repent. As Ibn Kathir said:
“He will repent from his ways and he will become basically the most pure person on earth.”
We also know that in the times that he will come, the Prophet said that:
The time shall be of the worst of times, and that the entire earth shall be filled with evil and injustice. (Abu Dawud)
And that is something that very rarely happens that the entire earth you see fitna and corruption, and you see Muslim blood being shed, and you see the Muslims being humiliated everywhere you look hardly ever has this occurred in the Ummah.
That is one of the times that we can expect Al-Mahdi figure to come.