Musnad Abu Ya'la al-Mawsili 154:180

Shayban narrated to us, Jarir ibn Hazim narrated to us, from Nafi’, from Ibn Umar, that a man called out to the Messenger of Allah (may Allah bless him and grant him peace) while he was on the pulpit and said: "O Messenger of Allah, how is the night prayer?" The Messenger of Allah (may Allah bless him and grant him peace) said, gesturing with his finger, describing it: "Two by two. Then, if you fear the morning, pray one rak'ah; it will make your prayer odd for you."