Musannaf Ibn Abi Shayba 3:4429

Hushaym narrated to us, he said: Muhammad bin Sa’id, Ibn ‘Awn, and others informed us, from Nafi’, from Ibn ‘Umar, that a man asked the Prophet, may Allah bless him and grant him peace, about the night prayer. He said: "Two by two. So if you sense the morning or fear the morning, pray one rak'ah; it will make your prayer odd (witr) for you."