Musnad Ahmad 27:950

Abu Sa'id, the freed slave of Banu Hashim, narrated to us, Layth narrated to us, 'Asim narrated to us, from 'Abdullah ibn Shaqiq, who said: I asked Ibn 'Umar about the night prayer. Ibn 'Umar said: A man asked the Prophet, peace and blessings of Allah be upon him, about the night prayer while I was between them. He said: "The night prayer is two by two. If you fear the dawn, then hasten the dawn with one rak'ah or two rak'ahs before the morning prayer."