Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from Hafsah (may Allah be pleased with her) that when the call to the Fajr prayer was given, the Prophet (peace and blessings be upon him) would perform two prostrations (rak'ahs) before the obligatory prayer and keep them light.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19682