What is the opinion of Sunnis about Mahdi (a.s.)?
Both Shiah and Sunnis believe that the Messenger of Allah (s.a.w.) has given good news about his advent and appearance and said to his companions: “Allah will make him appear on last days.” There are many traditions on these lines that we just mention some of them which are well authorized and authentic with Sunnis:
In Sunnan of Abu Davud it has been narrated that: “The Messenger of Allah (s.a.w.) said: ‘If there were to remain in the life of the world but one day, God would prolong that day until He sends in it a man from my household. His name will be the same as my name and his father’s name will be the same as my father’s name. He will fill the earth with equity and justice as it was filled with oppression and tyrancy.’” 
In Sunnan of ibn Majah it has been said that: “The Messenger of Allah (s.a.w.) said: ‘We are the household that God preferred for us the Other world to this world, and truly the household becomes homeless and suffers many hardships after me until a nation who has black flags rises up from the east and desires for charity but they are not given, so they fight and overcome and they are given whatever they want but this time they refuse to accept it until they leave the government to a man of my household that will fill the earth with equity as it was filled with oppression”. 
Ibn Majah has also said in Sunnan that: “The Messenger of Allah (s.a.w.) said: “Mahdi is from us, the household, Mahdi is from Fatima’s children.” And said: “Mahdi will appear among my nation, if his governing period be short, it will not be less than seven or nine years. And that my nation during his governing will enjoy such divine gifts and live in so much pleasure that has not occurred at any time. Gifts will be so increased and properties will be so stored up that if anybody says: “O Mahdi! Give me”, he will say: “Take it.” 
In Sahih of Tirmizi it has been said that: “A man from my household will govern whose name is the same as my name and truly if there were to remain in the life of the world but one day God would prolong that day until he governs.” And said: “The world is not finished until a man from my household governs Arabs whose name is the same as my name.” 
In his Sahih, Imam Bokhari has narrated that: “Ibn Bakir quoted from Yunis from ibn Shahab from Nafi Muwlay Qatadah Ansari that Abu Horayrah said: The Messenger of Allah (s.a.w.) said: ‘How will be you if the son of Maryam be appeared among you while your Imam is from yourself.’” 
In Fath al-Bari, Hafiz says: “It has been handed down by tradition that Mahdi is from this nation and Isa ibn Maryam will appear and say a prayer behind him.” 
In Sawaiq al-Muhraqah ibn Hijr Hiythami says: “Traditions in relation to the appearance of Mahdi are so many and has frequently been narrated.” 
Many of contemporaneous narrators have also narrated traditions regarding Imam Mahdi, for example in Al-Aqayid Al-Islamiyah, Sayyid Sabiq who is jurisconsult (mufti) of Ikhwan al-Muslimin has said that: “Belief in Mahdi is from Islamic beliefs that should be confirmed.”
Shiites have also narrated so many traditions concerning Mahdi (a.s.) that it has been said about it: “it has been quoted so many traditions in relation to His Highness Mahdi (a.s.) from the Messenger of Allah (s.a.w.) that has not occurred about any other case. 
 Sunnan of Abu Davud (2), p. 422.
 Sunnan of ibn Majah (2), tradition No. 4082 and 4087.
 Sunnan of ibn Majah (2), tradition No. 4086.
 Sahih of Tirmizi (4), tradition No. 2230 and 2232.
 Sahih of Bokhari (4), p.143.
 Fath al-Bari (5), p.362.
 Sawaiq al- Mohraqah (2), p. 211.
 With truthful people, p. 406.