Musnad Ahmad 27:1054

Muhammad ibn Ja’far narrated to us, from Shu’bah, from Khalid, from ‘Abdullah ibn Shaqiq, from Ibn ‘Umar, from the Prophet (peace and blessings of Allah be upon him) that he said: "The night prayer is two by two. So if you fear the morning, then prostrate one prostration and two rak'ahs before the morning."