Question about the messiah in Islam
Question

Who is the messiah of islam?
Praise be to Allah.The Muslims do not pin their hopes on someone who will save or deliver them, as is the case with the Jews, the Christians and the Raafidis (Shi’ah). Islam is complete and its rulings are complete. Perhaps what is meant in the question is the Mahdi. He will be one of the reformers of this ummah through whom Allah will fill the earth with justice and fairness after it was filled with injustice and unfairness. Like other reformers, he will follow the Quran and Sunnah and will rule according to them. 
Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him) said: 
The issue of the Mahdi is well known and the ahaadeeth which speak of him are numerous and are mutawaatir and support one another. More than one of the scholars has narrated that they are mutawaatir in meaning because of their many isnaads, many narrators and many versions. They indicate that this person whose coming is promised is something real and true. His name will be Muhammad ibn ‘Abd-Allah al-‘Alawi al-Hasani, and he will be a descendent of al-Hasan ibn ‘Ali ibn ‘Abi Taalib (may Allah be pleased with him). By the mercy of Allah towards this ummah, this imam will appear at the end of time and will establish justice and truth and do away with injustice and oppression. Through him Allah will spread justice among the ummah and guide the people. 
I have studied many of the ahaadeeth about him and I saw, as al-Shawkaani and others said, and as Ibn al-Qayyim and others said, that some are saheeh, some are hasan, and some are da’eef (weak) and manjabir (may be proven if there is corroborating evidence), containing fabricated reports. Those which have sound isnaads are sufficient for us, whether they are saheeh in and of themselves (saheeh li dhaatihi) or saheeh because of the existence of corroborating reports (saheeh li ghayri), or hasan in and of themselves (hasan li dhaatihi) or hasan because of the existence of corroborating reports (hasan li ghayrihi). By the same token, if the da’eef (weak reports) corroborate and support one another, then that constitutes evidence according to the scholars. So there are four categories of acceptable hadeeth according to the scholars: saheeh li dhaatihi (saheeh in and of itself), saheeh li ghayrihi (saheeh because of the existence of corroborating reports), hasan li dhaatihi (hasan in and of itself) and hasan li ghayrihi (hasan because of the existence of corroborating reports). 
This has to do with reports other than those which are mutawaatir. All the mutawaatir reports are acceptable, whether they are mutawaatir in wording or in meaning. The ahaadeeth which speak of the Mahdi are mutawaatir in meaning. And the trustworthy scholars have stated that they are sound and mutawaatir. 
We have seen the scholars state many things to be sound on the basis of less evidence than that. The truth of the matter is that the majority of scholars – if not all of them – are agreed that the Mahdi is something proven: he is real and will appear at the end of time. 
With regard to those scholars who hold odd views concerning this matter, no attention is to be paid to what they say on this matter. 
Al-Radd ‘ala man kadhaba bi’l-Ahaadeeth al-Saheehah al-Waaridah fi’l-Mahdi by Shaykh ‘Abd al-Muhsin al-‘Abbaad, p. 157. 
Shaykh Ibn Baz was commenting on a lecture given by Shaykh al-‘Abbaad. 
There follows some information about the Mahdi: 
His name will be Muhammad ibn ‘Abd-Allah. 
His description: He will have a high forehead and a hooked nose. 
Al-Qaari: Sharh Mishkaat al-Masaabeeh, 10/175.
When he will appear: At the end of time, shortly before the Hour begins. After him the Messiah ‘Eesa ibn Maryam (Jesus the son of Mary) will descend. 
There follow some of the ahaadeeth that have been narrated concerning him: 
It was narrated that Abu Sa’eed al-Khudri said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “The Mahdi is from me, with a high forehead and a hooked nose. He will fill the earth with fairness and justice as it was filled with unfairness and injustice, and he will reign for seven years.” Narrated by Abu Dawood, 4275; classed as hasan by al-Albani in Saheeh Abi Dawood.  
It was narrated that ‘Abd-Allah ibn Mas’ood said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “This world will not pass away until the earth is ruled by a man from among my family who whose name will be the same as mine.” Narrated by al-Tirmidhi, 2235. Shaykh al-Albani said: (it is) hasan saheeh. 
It was narrated that Jaabir (may Allah be pleased with him) said: I heard the Prophet (peace and blessings of Allah be upon him) say: “A group from among my ummah will continue to fight for the truth and prevail until the Day of Resurrection. Then ‘Eesa ibn Maryam (peace and blessings of Allah be upon him) will come down and their leader will say, ‘Come and lead us in prayer,’ but he will say, ‘No, you are leaders for one another, as an honour from Allah to this ummah.’” 
Narrated by Muslim, 156. 
The leader mentioned in this hadeeth is the Mahdi. 
For more reports about the Mahdi, please see question no. 1252. 
Note: 
Some people misunderstand the ahaadeeth about the Mahdi and the descent of ‘Eesa ibn Maryam, so they do not work for Islam or to call people to Allah because they are waiting for the Messiah to come down or for the Mahdi to appear. This is undoubtedly a mistake. The Muslims have to strive to make the word of Allah supreme, to make His religion prevail and to convey the message of truth to all people. We can imagine how much the Muslims would have declined and how weak they would have become if the earlier Muslims had understood this matter in this mistaken manner and acted accordingly. Would they have been able to defeat the Tatars and Crusaders, and conquer other lands? For more information please see question no. (21221) 
And Allah knows best.