Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: Whoever misses the Witr prayer due to sleep or forgetfulness, then whenever he remembers or wakes up, he should perform the Witr.
Hadith Referenceالفتح الربانی / كتاب الصلاة / 1238