Abu Nadrah (may Allah have mercy on him) said that Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) informed him that they asked the Prophet (peace and blessings be upon him) about witr, and he (peace and blessings be upon him) said: "The time of witr is before the morning prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4512