Hudhayfah reported that the Prophet (S) said, “Al-Mahdi will turn his face to Jesus when he descends as if water were dropping from his hair, and will say to him, ‘Go ahead and say the prayer.’ Jesus will say, ‘The prayer has been set up only for you.’ So, Jesus will pray behind a man who is among my sons. Sirat-al-mustaqim 257, a shia source.
While their Imaam will have advanced to pray the Fajr prayer with them, 'Eesa, the son of Mary may Allaah exalt his mention will descend [at the time of the Fajr prayer]. The Imaam will draw backward so that 'Eesa may Allaah exalt his mention would go forward and lead the people in prayer. However, 'Eesa may Allaah exalt his mention would put his hand between his shoulders and say to him: "Go forward and pray, as it is for you that the call for the prayer was called, so their Imaam would lead them in prayer."
Al-Munaawi may Allaah have mercy upon him said in his book entitled Faydh Al-Qadeer:
He (i.e. 'Eesa may Allaah exalt his mention) would descend at the time of Fajr prayer at the white minaret east of Damascus and he would find the Imaam (of prayer) Al-Mahdi wanting to establish the prayer. When he realizes that 'Eesa may Allaah exalt his mention is present, he would draw backward so that 'Eesa may Allaah exalt his mention would advance (to lead the people in prayer) but 'Eesa may Allaah exalt his mention would put him forward to lead the prayer and he would pray behind him. So, this is a great favor and honor for this nation. Thus, what is mentioned in this Hadeeth does not contradict what is mentioned in other narrations that 'Eesa may Allaah exalt his mention will lead Al-Mahdi in prayer. This was authoritatively asserted by Sa'd At-Taftaazaani who considered 'Eesa may Allaah exalt his mention to be better, as it is possible to reconcile (the narrations) that 'Eesa may Allaah exalt his mention would first pray behind the Mahdi in order to show that he ('Eesa) descended to be a follower of our Prophet Muhammad sallallaahu 'alayhi wa sallam ( may Allaah exalt his mention ) ruling by his [Prophet's] Sharee'ah (law). Then the Mahdi would be led by 'Eesa may Allaah exalt his mention on the basis of the principle that the one who is less important is led by the one who is better and more important.
Question: Does relinquishing lead of fajr salaat to 'Eesa bin Maryam by Mahdi an indication that ameer ul mumineen need not be from progeny of Ali and Fatima?