Shu'bah (may Allah have mercy on him) said: I heard Uqbah bin Hurayth say that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates 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. If you fear that the time of morning will come upon you, pray one rak'ah of witr." Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) was asked: "What is meant by two at a time?" He said: "Saying the salam after every two rak'ahs."
(b) In the narration of Adam: "Pray one rak'ah of witr." A man said to Sayyiduna Abdullah bin Umar (may Allah be pleased with them both): "What is meant by two at a time?" He said: "Saying the salam after every two rak'ahs."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4773