Sayyiduna Ibn Umar (may Allah be pleased with them both) said: Whoever prays at night, let him make witr his last prayer, for the Prophet (peace and blessings be upon him) commanded that. When dawn breaks, the time of the night prayer and of witr is over, because he (peace and blessings be upon him) said: "Witr is before dawn."
(b) In a narration of al-Fahham, he (peace and blessings be upon him) said: "Pray witr at dawn."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4516