What should one do who performed ablution for Fajr and does not remember whether he prayed it or not?

The fundamental principle in Sharia is that matters are built upon certainty, not doubt. So, whoever doubts having performed an act of worship, such as doubting whether they prayed or not, the default is that they have not performed it, and they must carry it out. The proof for this is the Prophet's (peace be upon him) hadith concerning someone who imagines finding something during prayer: "He should not leave until he hears a sound or finds a smell." The meaning of this is that certainty, which is ritual purity, should not be abandoned due to doubt. Therefore, whoever doubts a prayer, their responsibility remains until they perform it, as the scholars have stated.