It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said: "The night prayer is in sets of two rak‘ahs, and Witr is one rak‘ah in the last part of the night."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4878