Abu Mijlaz said: I asked Sayyiduna Ibn Umar (may Allah be pleased with them both) about witr, and he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "It is one rak'ah in the last part of the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4770