For our respected Sunni brothers, there are six authentic collections of traditions based on the Sunni standards for verifying the authenticity of a tradition. These six books are: Sahih al-Bukhari, Sahih Muslim, Sahih al-Tirmidhi, Sunan Ibn Majah, Sunan Abu Dawud, and Sahih al-Nisa'i. I just quote few traditions from these six books to prove that a knowledgeable Sunni brother/sister can NOT deny that:
 
Mahdi is going to come in the last days to make a universal Government,
Mahdi is from the Ahlulbait of Prophet,
Mahdi is from the children of Fatimah (p.b.u.h.), the daughter of Prophet,
Mahdi is different than Jesus (the messiah),
Jesus will be one of the followers of Imam Mahdi and prays behind him.
The following are only some of the traditions out of many, about Imam Mahdi, and are ALL traditions that the Sunnis admit to their authenticity and existence:
 
The Prophet (p.b.u.h.&h.f.) said: "Even if the entire duration of the world's existence has already been exhausted and only one day is left before Doomsday (Day of Judgment), Allah will expand that day to such a length of time, as to accommodate the kingdom of a person out of my Ahlulbait who will be called by my name. He will then fill out the earth with peace and justice as it will have been full of injustice and tyranny before then."
 
Sunni References:
Sahih Tirmidhi, V.2, P.86, V.9, PP. 74-75
Sanan Abi Dawud, V2, P7
Musnad Ahmad Ibn Hanbal, V1, P376 & V3, P63
Mustadrak al-Sahihain, by al-Hakim, V4, P557
Al-Majma', by Tabarani, P217
Tahdhib al-Thabit, by Ibn Hajar al-Asqalani, V9, P144
Sawaiq al-Muhraqa, Ibn Hajar al-Haythami, P167
Fathul Bari, by Ibn Hajar al-Asqalani, V7, P305
al-Tathkirah, by al-Qurtubi, P617
al-Hawi, by al-Suyuti, V2, pp165-166
Sharh al-Mawahib al-Ladunniyyah, by al-Zurqani, V5, P348
Fathul Mughith, by al-Sakhawi, V3, P41
al-Hafidh Abul-Hasan Muhammad Ibn al-Husayn al-Sijistani al-Aburi
al-Shafi'i (d. 363/974). (who said the above hadith is related by numerous authorities and were spread far and wide by many narrators) and also in the works of Ibn Habban, Abu Nua'ym, Ibn Asakir, etc.
 
Also:
 
The Prophet (p.b.u.h.&h.f.) said: "Al-Mahdi is one of us, the members of the household (Ahlulbait)."
 
Sunni reference: Sunan Ibn Majah, V2, Tradition #4085.
 

As we see Imam Mahdi is from the Ahlulbait of Prophet Mohammad, so he can not be Jesus (the Messiah; al-Maseeh). Mahdi and Messiah are two different personalities but they come at the same time, Mahdi as Imam and Jesus as his follower. The following tradition clearly mentions that Imam Mahdi is one of the children of the daughter of Prophet Muhammad (PBUH&HF):
 
The Messenger of Allah said: "Al-Mahdi is one of the children of Fatimah (the Prophet's daughter)."
 
Sunni reference: Sunan Ibn Majah, Vol.2, Tradition #4086.
 

Also:
 
The Prophet (p.b.u.h.&h.f.) said: "We the children of Abd Al-Mutalib are the Masters of the inhabitants of Heaven: Myself, Hamza (RA), Ali (p.b.u.h.), Ja'far (RA), Hassan (p.b.u.h.), Hussain (p.b.u.h.), and Al-Mahdi (a.t.f.s.)."
 
Sunni reference: Sunan Ibn Majah, V2, Tradition #4087.
 
The Prophet (p.b.u.h.&h.f.) said: "The Mahdi will appear in my Ummah. He will appear for a minimum of 7 or a maximum of 9 years; in that time, my Ummah will experience a bountiful favor like never before. It shall have a great abundance of food, of which it need not save anything, and the wealth at that time is in great quantities, such that if a man asks the Mahdi to give him some, and the Mahdi (a.t.f.s.) will say: Here! Take!"
 
Sunni reference: Sunan Ibn Majah, Vol.2, Tradition #5083.
 
Remark: According to Shia sources, the Government of Peace and equality that Imam Mahdi will establish will last hundreds of years with no rival, and then the day of Judgment will be set. What is mentioned in the above tradition as 7 or 9 years is related to the length of time that Imam Mahdi will fight to conquer the world when he starts his mission.
 
The Prophet (p.b.u.h.&h.f.) said: "We (I and my family) are members of a household that Allah (SWT) has chosen for them the life of the Hereafter over the life of this world; and the members of my household (Ahlulbait) shall suffer a great affliction and they shall be forcefully expelled from their homes after my death; then there will come people from the East carrying black flags, and they will ask for some good to be given to them, but they shall be refused service; as such, they will wage war and emerge victorious, and will be offered that which they desired in the first place, but they will refuse to accept it, until a man from my family (Ahlulbait) appears to fill the Earth with justice as it has been filled with corruption. So whoever reaches that (time) aught to come to them even if crawling on the ice/snow."
 
Sunni ref: Sunan Ibn Majah, Vol.2, Tradition #4082, also in the History Tabari
 

The Messenger of Allah said: "The world will not perish until a man among the Arabs appears whose name matches my name."
 
Sunni reference: Sahih Tirmidhi, Vol.9, P.74.
 
Also:
 
The Messenger of Allah said: "Mahdi from my family will bring about a revolution and will fill the world with justice and equity before which it was filled up with injustice and inequity."
 
Sunni references:
 Musnad Ahmad Ibn Hanbal, V1, P84
Jami'us Sagheer, by al-Suyuti, PP 2,160
al-Urful Vardi, by al-Suyuti, P2
Kanzul Ummal, V7 P186
Aqd al-Durar Fi Akhbaar al-Mahdi al-Muntazir, V12, Chapter 1,
al-Bayan fi Akhbar Sahib al-Zaman, By Ganji Shafi'i, Chapter 12
al-Fusool al-Muhimmah, by Ibn Sabbagh Maliki, Chapter 12
Arjahul Matalib, by Ubaidallah Hindi hanafi, P380
Muqaddimah, by Ibn Khaldoon, P266
 
Also Ahmad Ibn Hanbal narrated that:
 
The Prophet (p.b.u.h.&h.f.) said: "Allah will bring out from concealment Mahdi from my Family and Progeny 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 eradicate tyranny and oppression."
 
Sunni reference: Musnad Ahmad Ibn Hanbal, v1, p99
 

Ibn Majah in his Sunan quotes Mohammad Ibn Hanafiyyah and Imam Ali saying that the Holy Prophet (p.b.u.h.&h.f.) said:
 

"Mahdi is from our Ahlulbait, no doubt Allah will enforce his Amr (appearance) within a night (i.e., his coming is very unpredicted)."
 
Sunni reference: Sunan Ibn Majah, V2, P269
 

The more recent fatwa in this issue is given in Mecca by the Muslim World League (Rabitatul `Alamul Islami) on Oct. 11, 1976 (23 Shawwal 1396). This fatwa states that more than twenty companions narrated traditions concerning al-Mahdi, and gives a list of those scholars of Hadith who have transmitted these narrations, and those who have written books on al-Mahdi. The fatwa states:
 
"The memorizers (Hafiz) and scholars of Hadith have verified that there are authentic (sahih) and acceptable reports among the traditions related to al-Mahdi. The majority of these traditions are related through numerous authorities (Mutawatir). There is no doubt that the status of those reports are Sahih and Mutawatir. (They have also verified) that the belief in Mahdi is obligatory, and that it is one of the beliefs of Ahl al-Sunnah wal Jama'a. Only those ignorant of the Sunnah and innovators in doctrine deny it.
 
For the transcription and reproduction of this fatwa, see, among others, the Introduction of al-Ganji al-Shafi'i, in the book named "al-Bayan," Beitut, 1399/1979, PP 76-79 and in Appendix.
 
Among Shiite scholars, I would like to mention the masterpiece of Lutifullah Al-Safi al-Golpaygani, who compiled an encyclopedia called "Muntakhab al-Athar". In this book, there is a comprehensive narration of the traditions relating to the appearance of the Mahdi (a.t.f.s.). He has used over 60 Sunni sources, including their Six Authentic Books of Tradition; and over 90 Shiite sources to illuminate the fact that Al-Mahdi is NOT a fabricated event.
 
The Prophet (P.B.U.H.&H.F.) said: "He who dies not knowing the Imam of his era, has died the death of Al-Jahiliyya (Days of Ignorance; the era before Islam)."