Musnad Ahmad 27:1343

Muhammad ibn Ubayd narrated to us, Ubaydullah narrated to us, from Nafi’, from Ibn Umar, who said: A man asked the Messenger of Allah, peace and blessings of Allah be upon him, while he was on the pulpit, about the night prayer. He said: "Two by two. So if one of you fears that morning may come, let him pray one, and it will make odd for him what he has prayed."