It is narrated from Hazrat Aisha (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) did not observe any voluntary prayer more regularly than the two rak'ahs before Fajr among the optional prayers.
Hadith Referenceمسند احمد / مسند النساء / 24271
Hadith Gradingحکم دارالسلام:إسناده صحيح وهو مكرر: 24167