It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that once the Prophet (peace and blessings be upon him) said: The night prayer consists of two rak'ahs at a time, and when you fear that morning is near, then add one rak'ah to these two and make your last prayer the Witr.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6008