Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from Hafsah (may Allah be pleased with her) that when the mu'adhdhin finished the call to the Fajr prayer and fell silent, and the (true) dawn had appeared, the Prophet (peace and blessings be upon him) would pray two light rak'ahs before the prayer was established.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19683