Musnad Abu Ya'la al-Mawsili 210:23

Zuhayr narrated to us, Yahya bin Sa'id narrated to us, from Ubaydullah, who said: Nafi' informed me, from Ibn Umar, who said: Hafsa informed me that the Prophet, peace and blessings of Allah be upon him, used to pray two light prostrations (rak'ahs) when the dawn broke.