There were many Hadiths narrated concerning Al-Mahdi, ranging between being fabricated, weak, sound, and authentic. However, the authentic Hadiths narrated in that respect are very few. Contemplating over such Hadiths, Muslim jurisprudents concluded what should be believed in that regard: that is, the emergence of Al-Mahdi is a fact that should not be denied; he will be born before the end of this life and will grow up to fill the earth with justice as it has been filled with oppression. This does not require Muslims to wait anxiously for the emergence of Al-Mahdi because his emergence will not abrogate any ruling in the Islamic Shari`ah. Al-Mahdi cannot be regarded as a prophet, since Prophet Muhammad (peace and blessings be upon him) is the Last Prophet of Allah, and no prophet will be sent after him.

In this regard, the prominent Saudi scholar Sheikh Salman Ibn Fahd Al-`Udah, states the following: “More than one hundred Hadiths were narrated about Al-Mahdi. Those Hadiths ranged between being fabricated, weak, sound, and authentic; the authentic ones are very few.
Of such Hadiths are the following:
1. `Ali Ibn Abi Talib quoted the Prophet (peace and blessings be upon him) as saying: “Al-Mahdi is one of us, the clan of the Prophet (peace and blessings be upon him). Allah will reform him in one night.” (Reported by Imam Ahmad and Ibn Maqah).
2. There were three Hadiths narrated on the authority of Abu Sa`id Al-Kudri (may Allah be pleased with him). Such Hadiths were reported by Al-Hakim; and some of them were reported by Abu Dawud, At-Tirmidhi, Ibn Maqah, Imam Ahmad, and others. Of these Hadiths, At-Tirmidhi reported that the Prophetsaid: “Al-Mahdi is from my Ummah; he will be born and live to rule five or seven or nine years. (If) one goes to him and says: ‘Give me (a charity), he will fill one’s garment with what one needs.’”
3. Abu Dawud also reported a Hadith about Al-Mahdi that the Prophet (peace and blessings be upon him) said: “Al-Mahdi will be of my stock, and will have a broad forehead, a prominent nose. He will fill the earth with equity and justice as it was filled with oppression and tyranny, and he will rule for seven years.”
4. There is also the Hadith narrated by Thawban (may Allah be pleased with him) in which the Prophet (peace and blessings be upon him) said: “If you see the black banners emerging from Khurasan, seek to join their supporters even if creeping, because among them will be caliph Al-Mahdi.” That Hadith was reported by Al-Hakim and Ahmad. But the chains of narration of that hadith were all unauthentic, though some Hadith scholars rendered it sound in general.
Some people stated that the aforementioned Hadith is cited to support that Al-Mahdi will emerge from among the Abbasid State. That Hadith might be fabricated altogether or its words might be distorted so as to support the Abbasid State.
Added to the above, I would like to say that there are Hadiths reported about Al-Mahdi that are true in general, but most of them do not reach the degree of authenticity. It may be only one of these Hadiths that is regarded authentic. Moreover, it is only a few of them that are considered sound, while most of them are weak.
Many scholars tackled the subject of Al-Mahdi, among whom was Na`im Ibn Hamad. He wrote about Al-Mahdi in his book Al-Fitan. Although Na`im was an Imam of Sunnah, Ad-Darqatani, Adh-Dhahabi and Ibn Hajar mentioned that there were some weak and fabricated Hadiths reported by him.
Others who wrote about Al-Mahdi are Abu Na`im Al-Asfahani and Yusuf Ibn Yahyah As-Salami, whose book is named “`Uqad Ad-Durar. Ibn Khaldun also wrote about that subject in his well-known book Al-Muqaddimah. He stated: “It is known among the majority of Muslims that Al-Mahdi is a fact.” He further stated that the majority of scholars and Imams emphasized the Hadiths reported about Al-Mahdi in general, though many of these Hadiths were controversial.
there are also many recent scholars who believed in the truthfulness of the Hadiths reported about Al-Mahdi in general.
However, some scholars denied all the Hadiths reported in that regard. Of those scholars was Mujahid, who claimed that Al-Mahdi would be Jesus, Son of Mary. In that respect, Ibn Maqah and Al-Hakim reported a weak Hadith to the effect that Al-Mahdi would be Jesus, Son of Mary.
among the recent scholars who believe the Hadiths about Al-Mahdi in general are Sheikh Muhammad Rashid Rida, Sheikh `Abdullah bin Zayd Aal-Mahmud, Muhammad Muhi Ad-Din `Abdul-Hamid and others.
Muslims who are of Ahl-us Sunnah (mainstream Muslims) believe that a man of the Prophet’s clan will be born before the end of this world and lead an ordinary life as any other one; he may commit mistakes and will need to be reformed like any other one. Then Allah will choose him to reunite the Muslims and guide them to the right path.
This is all what should be believed about Al-Mahdi. There is no religious text to the effect that it is a religious duty to wait anxiously for him. Moreover, Muslims should not believe anyone claiming that he is Al-Mahdi unless there is clear evidence to that effect, as many people have claimed to be so.
thus, Muslims are required not to hasten to believe anyone claiming to be Al-Mahdi; they are to investigate and make sure of such a thing.
There is no ruling in the Islamic Shari`ah that has to do with the emergence of Al-Mahdi. That is to say, Muslims are not to delay fulfilling religious duties like the congregational prayers or Jihad or applying penalties prescribed in Shari `ah, etc., until Al-Mahdi comes and guides them in that respect. Muslims are to lead their lives normally — worship Allah, go to work, reform their society, learn, teach, etc., and if Al-Mahdi appears and they make sure that he is truthful, they are to follow him. This was the attitude of the Companions and those who truly followed in their footsteps.”