It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: Whoever sleeps without offering Witr or forgets it, he should offer it when he remembers or wakes up.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11395
Hadith Gradingحکم دارالسلام:حديث صحيح، عبدالرحمن بن زيد بن أسلم- وإن يكن ضعيفا- متابع