Sayyiduna Ibn Umar (may Allah be pleased with them both) narrated from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The night prayer is two rak'ahs at a time; then if you fear the coming of dawn, pray one rak'ah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13404