Musnad Abu Ya'la al-Mawsili 153:204

Ghassan ibn al-Rabi' narrated to us, from Thabit al-Taymi, from Tawus, from Ibn Umar, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The night prayer is two by two. So if you fear or dread the morning, then [pray] one."