The following are the collection of Sahih (authentic) Hadith regarding Mahdi.
Hadhrat Abdullah bin Mas'ood(RA) reports from the Prophet(SAW), who said:
The world will not come to pass until a man from among my family, whose name will be my name, rules over the Arabs.
(Tirmidhi Sahih, Vol. 9, P. 74; Abu Dawud, Sahih, Vol. 5, P. 207;
also narrated by Ali b. Abi Talib, Abu Sa'id, Umm Salma, Abu Hurayra)
The Prophet(SAW) said:
Allah will bring out from concealment al-Mahdi from my family and just before the day of Judgment; even if only one day were to remain in the life of the world, and he will spread on this earth justice and equity and will eradicate tyranny and oppression.
(Musnad Ahmad Ibn Hanbal, Vol. 1, P. 99)
Hadhrat Ali(RA) narrates that Rasulullah(SAW) said:
Even if only a day remains for Qiyamah to come, yet Allah will surely send a man from my family who will fill this world with such justice and fairness, just as it initially was filled with oppression.
Ali b. Abi Talib(RA) has related a tradition from the Prophet(SAW) who informed him:
The promised Mahdi will be among my family. God will make the provisions for his emergence within a single night.
(Ibn Majah, Sahih, Vol. 2, P. 519)
Hazrat Umme Salmah(RA), Prophet's wife, narrates that she heard the Prophet(SAW) say:
The promised Mahdi will be among my progeny, among the descendants of Fatima.
(Abu Dawud, Sahih, Vol. 2, P. 207; Ibn Majah, Sahih, Vol. 2, P. 519)
The Mahdi will be of my family, of the descendants of Fatima (the Prophet's daughter).
(Sunan Ibn Majah, Vol. 2, Tradition No. 4086)
The Prophet(SAW) taught:
Al-Mahdi is one of us, the members of the household (Ahlul-Bayt).
(Sunan Ibn Majah, Vol. 2, Tradition No. 4085)
Abu Sa'id al-Khudari(RA) narrated that the Prophet(SAW) said:
Our Mahdi will have a broad forehead and a pointed (prominent) nose. He will fill the earth with justice as it is filled with injustice and tyranny. He will rule for seven years.
(Abu Dawud, Sahih, Vol. 2, p. 208; Fusul al-muhimma, p. 275)
Hadhrat Abu Saeed Khudri(RA) relates that Rasulullah(SAW) said:
Al Mahdi will be from my progeny. His forehead will be broad and his nose will be high. He will fill the world with justice and fairness at a time when the world will be filled with oppression. He will rule for seven years.
Hadhrat Umme Salmah(RA) narrates that Rasulullah(SAW) said:
After the death of a Ruler there will be some dispute between the people. At that time a citizen of Madina will flee (from Madina) and go to Makkah. While in Makkah, certain people will approach him between Hajrul Aswad and Maqaame Ibraheem, and forcefully pledge their allegiance to him.
Thereafter a huge army will proceed from Syria to attack him but when they will be at Baida, which is between Makkah and Madina, they will be swallowed into the ground.
On seeing this, the Abdaals of Shaam as well as large numbers of people from Iraq will come to him and pledge their allegiance to him. Then a person from the Quraish, whose uncle will be from the Bani Kalb tribe will send an army to attack him, only to be overpowered, by the will of Allah. This (defeated) army will be that of the Bani Kalb. Unfortunate indeed is he who does not receive a share from the booty of the Kalb. This person (Imam Mahdi) will distribute the spoils of war after the battle. He will lead the people according to the Sunnat and during his reign Islam will spread throughout the world. He will remain till seven years (since his emergence). He will pass away and the Muslims will perform his Janazah salaat.
The holy Prophet(SAW) said:
A group of my Ummah will fight for the truth until near the day of judgment when Jesus, the son of Marry, will descend, and the leader of them will ask him to lead the prayer, but Jesus declines, saying: "No, Verily, among you Allah has made leaders for others and He has bestowed his bounty upon them.
It is reported from Abu Hurayra(RA) that the Prophet(SAW) said:
What will be your reaction when the son of Mary (Jesus) descends and your Imam is from among yourselves?
(Sahih Muslim, bab nuzul 'isa, Vol. 2; Sahih Bukhari, kitab bad' al-khalq wa nuzul 'isa, Vol. 4)
The holy Prophet(SAW) said:
What would be your situation if the Son of Mary (i.e. Jesus) descends upon you and your Imam is from among you?
(Bukhari, kitabul-Anbiya, Chapter Nuzul Isa bin Maryam)
According to these reliable, authentic, and universally accepted narrations, Mahdi will:
1. Be from among the family of Prophet(SAW), among the descendants of Fatima(RA);
2. Have a broad forehead and pointed noise;
3. Appear in one night;
4. Appear just before the day of judgment;
5. Have same name as hazrat Muhammad(SAW);
6. Escape from Madina to Makkah where people will pledge allegiance to him;
7. Receive pledge and help of Iraqi people;
8. Fight in battles;
9. Rule over the Arabs for seven years according to Sunnah;
10. Spread justice and equity on earth;
11. Eradicate tyranny and oppression;
12. Lead a prayer in Mekkah which Jesus(pbuh) will follow in;
13. NOT be the same individual as the Promised Messiah (Jesus).