Before this We wrote in the Psalms, after the message (had been given to Moses): My servants, the righteous, shall inherit the earth”
(The Holy Quran, 21:105)

All Islamic groups agree on a man who appears at the end of time to fill the world with justice and equity and establish the government of righteousness over all parts of the earth as a testimony to this verse:

“Before this We wrote in the Psalms, after the message (had been given to Moses): My servants, the righteous, shall inherit the earth” (Quran, 21:105),

and also this verse:

“And We wished to be gracious to those who were being oppressed in the land, to make them leaders (in faith) and to make them the heirs” (Quran, 28:5),

and also this verse:

“They would fain extinguish Allāh`s light with their mouths, but Allāh will not allow but that His light should be perfected, even though the unbelievers may detest (it). It is He who has sent His Prophet with guidance and the religion of truth to proclaim it over all religions, even though the pagans may detest (it)” (Qur'an, 9:32-33).

The Chosen One clarified that this awaited man is from among his own family; he said, “The world will not come to an end before the Arabs are ruled by a man from among my own family whose name is similar to mine..."
[Al-Tirmidhi, Sahīh, Vol. 9, p. 74. Abu Dāwūd, Vol. 2, p. 7. Ahmad, Musnad, Vol. 1, p. 376]

Abu Sa`īd al-Khudri is quoted as having said that the Messenger of Allah said, “The Hour shall not come till the earth is filled with oppression, suppression and animosity, then will come out of my family one who will fill it with equity and justice after being filled with oppression and transgression."
[Mustadrak al-Sahīhayn, Vol. 4, p. 557. Ahmad, Musnad, Vol. 3, p. 36.]

Abu Hurayra is quoted as having cited the Messenger of Allāh as saying, “If only one day remained of the life in this world, Allāh, the most Exalted One, the most Great, would have prolonged it till a man from among my Ahl al-Bayt rules the Daylam Mountain and Constantinople.”
[Ibn Mājah, Sahīh, in a chapter about ijtihād.]

Umm Salamah is quoted as having cited the Messenger of Allāh saying, “Al-Mahdi is from among my offspring, from the offspring of Fātima.” The Messenger of Allāh said that Jesus, peace be upon him, would appear at the end of time and would pray behind al-Mahdi. Abu Hurayra quotes the Messenger of Allah as saying, “How will you be if the son of Maryam (Mary) descends among you and your own Imām is your king?!
[Muslim, Sahīh, Vol. 1, p. 373 in a chapter about Jesus son of Mary descending (Dār al-Sha`ab edition).]

Al-Hāfidh, in Sharh Sahīh al-Bukhāri, has said, “Narratives are consecutively reported that al-Mahdi is from this nation, and that Jesus son of Mary will descend and pray behind him.”
[Fath al-Bāri, Vol. 5, p. 362.]

The Fiqh Assembly of the Muslim World League (Rabitat al-Aalam al-Islami) issued the following fatwa (verdict) dated May 31, 1976 about the Awaited Mahdi:

“Al-Mahdi, peace be upon him, is Muhammad ibn Abdullāh al-Hasani al-Alawi al-Fatimi al-Mahdi, the Awaited One. The time of his appearance is at the end of time, and it [appearance] is one of the signs of the Great Hour. He shall come out from the west, and he will receive the oath of allegiance in Hijaz, in Venerable Mecca, between the Rukn and Maqam [of Ibrahim], between the Honored Ka`ba and the fixed Black Stone. He will appear when there is a great deal of corruption, when disbelief spreads and when people oppress, and he will fill the earth with justice and equity just as it was filled with injustice and oppression. He shall rule the entire world and everyone will be his subject, once through conviction and once through war. He shall rule the earth for seven years, and Jesus, peace be upon him, will descend after him and kill the Dajjāl [anti-Christ] or descends with him and helps him kill him at the Ludd Gate on the land of Palestine. And he is the last of the twelve righteous caliphs about whom the Prophet spoke as recorded in the Sahīh books... The belief in the appearance of al-Mahdi is obligatory, and it is one of the tenets of the followers of Sunnah and Jamā`ah and is not denied except by one who is ignorant of the Sunnah and one who brings an innovation into the creed.”
[Mu`āmarat al-Mut ājirīn bid-Dīn [plot of those who trade in religion], p. 29.]

Hence, Sunnis agree with the Shī`ahs that Imām al-Mahdi (`atfs) is the last of the Twelve Caliphs about whom the Messenger of Allah gave the glad tidings, and both parties agree on most other points relevant to the Awaited Imām.

As regarding their differences in his regard, these are:

FIRST: Most Sunnis believe Imām al-Mahdi will be born at the end of time, while the Shī`ahs believe he was born in 255 A.H. (869 A.D.) to his father Imām al-Hasan al-Askari , the twelfth among the Imāms from among Ahl al-Bayt, but Allah Almighty veiled him from the eyes for a wisdom which He decreed, and he remains alive and will come out at the end of the time.

SECOND: Sunnis, as in the above-quoted fatwa, believe al-Mahdi is a descendant of al-Hasan and the name of his father is Abdullāh based on a narrative recorded by them: “... His name shall be similar to mine, and the name of his father similar to my father`s,” whereas Shī`ahs believe al-Mahdi descends from Imām al-Husayn and was born to his father al-Hasan al-Askari. The latest narrative they report as follows: “... His name shall be similar to mine, and his father`s name similar to my son`s”, a reference to the Prophet`s grandson al-Hasan.
Some Sunni writers tried to criticize and charge the Shī`ahs for their belief in the birth of the Awaited Imām and in his holding the reigns of Imāmate at the age of five. This criticism is mostly rendered to fanaticism due to their own conviction. Anything contrary to their conviction or to what they have been accustomed to or have inherited, they immediately pass their judgment on it as “invalid” without looking into the arguments of others.

The response to this is:

There are many Sunni scholars who believe al-Mahdi is Muhammad ibn al-Hasan al-Askari (`a), and that he is still alive [and in occultation] till Allāh permits him to come out. They, thus, are in agreement with what the Imāmite Twelver Shī`ahs say. Among these scholars are:

• Muhyi ad-Dīn ibn al-`Arabi in Futūhāt al-Makkiyya.
• Sibt ibn al-Jawzi in his book Tadhkirat al-Khawāss.
• Abd al-Wahhāb al-Sha`rāni in his book Aqā`id al-Akābir.
• Ibn al-Khashshāb in his book Tawarīkh Mawālīd al-`Aimma w Wafiyyātihim.
• Muhammad al-Bukhāri al-Hanafi in his book Fasl al-Khitāb.
• Ahmad ibn Ibrāhīm al-Balādhuri in his book Al-Hadīth al-Mutasalsil.
• Ibn al-Sabbāgh al-Māliki in his book Al-Fusūl al-Muhimma.
• The man of knowledge Abd al-Rahmān in his book Mir`āt al-Asrār.
• Kamāl ad-Dīn ibn Talhah in his book Matālib al-Su`ūl fī Manāqib al-Rasūl.
• Al-Qandūzi al-Hanafi in his book Yanābī` al-Mawadda.

How could a man live for so long?

There is no evidence from the Sharī`ah proving the opposite. The occultation of the Awaited Imām has many similar miracles about which the Holy Qur'an informs us. Noah, peace be upon him, remained in his people for 950 years calling them to the way of Allāh: “... and he stayed among them a thousand years less fifty” (Qur'an, 29:14). He, of course, lived longer than that. The Fellows of the Cave remained asleep for 309 years. Allāh Almighty raised Jesus, peace be on him, to Him, saved him from being killed and will send him back to this world at the end of time. Al-Khidir, too, peace be upon him, remains alive veiled from our eyes.

As regrading the tender age of al-Mahdi when he received the reigns of Imāmate following the death of his father, al-Hasan al-Askari, the eleventh in the series of the Imāms from among Ahl al-Bayt, there are similar and even greater miracles than that. Allāh made Jesus son of Mary, peace be upon him, a prophet even as he was a suckling infant in the cradle: “But she pointed to the babe. They said, `How can we talk to one who is a child in the cradle?!` He said, `I am, indeed, a servant of Allāh: He has given me Revelation and made me a prophet...`” (Qur'an, 19:29-30). And Allāh granted authority to Yahya (John the Baptist) while still a child: “`O Yahya! Take hold of the Book firmly.` And We granted him wisdom even as a youth” (Qur'an, 19:12).

If anyone says that these miracles were for the prophets, we say that there is no evidence from the Sharī`ah pointing to miracles coming to a halt after the demise of the greatest Prophet. Miracles are not only for prophets. The fellows of the cave were not prophets. Even the master of devils, Iblis, Allāh extended his life-span till the Hour.