It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that once a man asked the Prophet (peace and blessings be upon him) about Witr while I and that man were walking. The Prophet (peace and blessings be upon him) said: "The night prayer consists of two rak'ahs at a time, and Witr is one rak'ah."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5537