Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that a man asked the Prophet (peace and blessings be upon him) about the night prayer, and he (peace and blessings be upon him) said: "Two rak'ahs at a time; when you fear the arrival of the morning, pray one rak'ah of witr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4766