All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
The authentic and well-known narrations state that Al-Mahdi will appear before the Anti-Christ. When Islamic conquests reach Constantinople, Satan will shout saying, “The Anti-Christ has emerged among your people”. [Muslim and others] When he (Anti-Christ) appears, he will stay 40 days then ‘Eesa (Jesus) will descend, kill the Anti-Christ and pray behind Al-Mahdi. This is mentioned in Saheeh Muslim and other books.
Allaah Knows best.