There are many Hadiths that indicate the emergence of Al-Mahdy. They were reported in many chains of transmission. A group of scholars said that these Hadiths are morally Mutawatir (a Hadith reported by a significant number of narrators throughout the chain of narration, whose agreement upon a lie is impossible) like
Abu Al-Hasan Al-Ajurry who is one of the scholars of the fourth century,
Al-Safariny in his book Lawami` Al-Anwar Al-Bahiyyah and
Al-Shawkany in a booklet that he called Al-Tawdih Fi Tawatur Ahadith Al-Mahdy Wa Al-Dajjal Wa Al-Masih. There are other famous signs mentioned in other Hadiths. (Part No. 3; Page No. 142) The most important of which is:
"He will spread justice on earth after it was full of injustice and oppression."
No one is permitted to assert that so-and-so is Al-Mahdy until other required signs that were pointed out by the Prophet (peace be upon him) in sound Hadiths become well-known. The most important of these signs is that:
"He will spread justice on earth..."
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.