Abu Mijlaz says: I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about the witr, and he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Pray one rak'ah in the last part of the night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12849