Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that a man asked the Messenger of Allah (peace and blessings be upon him) about the night prayer, and he (peace and blessings be upon him) said: "The night prayer is two rak'ahs at a time. When one of you fears the arrival of the morning, he should pray one rak'ah, making what he has prayed odd (witr)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4764