Al-Muttalib bin Abdullah bin Hantab narrated: A man asked Ibn Umar (may Allah be pleased with them both) about the witr prayer, and he said: "Pray witr with one rak'ah." The man said: I fear that people will say it is "butayra'" (cut off, incomplete). He said: "It is the Sunnah of Allah and the Sunnah of His Messenger," meaning that this is the Sunnah of Allah and the Sunnah of His Messenger, the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13374