It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "The night prayer consists of two rak'ahs at a time. When you fear that dawn is approaching, then add one rak'ah as witr to these two."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5454
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ : 473، م :729.