Musnad Ahmad 1153:991

Yahya ibn Ghaylan narrated to us, he said: Al-Mufaddal, meaning Ibn Fadalah, narrated to us, he said: Yazid ibn al-Had narrated to me, that Urwah ibn al-Zubayr used to narrate from Aisha, the wife of the Prophet, may Allah bless him and grant him peace, that she used to say: "When the dawn broke, the Messenger of Allah, may Allah bless him and grant him peace, would pray two light rak'ahs, then he would lie down on his right side."