Ibn Jurayj said: I said to Ata: Are the two rak'ahs of Fajr obligatory or voluntary? He said: Do you not know? Then he narrated to me from Ubayd bin Umayr the hadith of Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) was not more constant upon any of the voluntary prayers than upon the two rak'ahs of the morning, or of Fajr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4466