Please read the narartion in Sahih Muslim which sows The Holy Prophet siad ” there would be a caliph in the last (period) of my Ummah who would freely give handfuls of wealth to the people without counting it. I said to Abu Nadra and Abu al-‘Ala: Do you mean ‘Umar b. ‘Abd al-Aziz? They said: No (he would be Imam Mahdi).
Signs about Imam Mahdi (as) in Sahih Muslim
There are well known signs about Imam Mahdi’s re-appearance in Shia Sunni books. When Imam Mahdi (as) will show himself in Mecca. A group of army would be sent to fight with him. When that group of army will reach out side Medina to a plain ground, the army would be sunk in the ground.
This one of the proven sign of 12th Imam Mahdi (as).
Some people got confuse with this narration that it is about Ibn e Zubair but the narrater told this narration is not about Ibn e Zaubair army, Abdullah b. Safwan said: By God, it does not imply this army.
Please read another narration.