Salim bin Abdullah (may Allah have mercy on him) said that Abdullah bin Umar (may Allah be pleased with them both) said: A man from among the Muslims asked the Prophet (peace and blessings be upon him): "O Messenger of Allah! How is the night prayer?" 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." Abdullah bin Umar (may Allah be pleased with them both) used to say the salam after every two rak'ahs, then pray one rak'ah of witr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4767