Umm al-Mu'mineen Sayyidah Aisha (may Allah be pleased with her) narrates that she said: The Messenger of Allah (peace and blessings be upon him) used to perform the two Sunnah rak'ahs of Fajr (after the adhan) so lightly that I would wonder in my heart whether he (peace be upon him) had even recited Surah al-Fatihah in those rak'ahs.
Hadith Referenceالسنن المأثورة / باب ما جاء في صلاة الخوف / 69
Hadith Takhrijصحیح مسلم، المساجد ومواضع الصلاة، باب استحباب رکعتی سنة الفجر، رقم: 724۔