What should a person do who entered the mosque for the Fajr prayer, but had not prayed the Dawn prayer (Fajr), and found the Imam leading the Morning prayer (Subh)?

If the prayer is established, the worshiper should join the Imam and not preoccupy himself with supererogatory prayers, due to the Prophet's saying: "If the prayer is established, then there is no prayer except the obligatory one." If he has not prayed the Fajr supererogatory prayer, he should pray it after the obligatory prayer as a make-up, based on Qays's Hadith: "Are there two prayers together?" I said: "O Messenger of Allah, I had not performed the two rak'ahs of Fajr," He said: "Not so then." It is permissible to delay making it up until after sunrise and the disappearance of its redness.