It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever sleeps through his witr or forgets it, let him pray it in the morning or whenever he remembers it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4533