Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that Hafsah (may Allah be pleased with her) told me that the Prophet (peace and blessings be upon him) used to pray the two rak'ahs of Fajr when the dawn broke.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19636