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
Wabarah said: I asked Ibn Umar (may Allah be pleased with them both) about a person who leaves witr until the sun rises: should he pray it? He said: What do you think, if you left the morning prayer until the sun rose, would you pray it? The narrator said: I said: Then what? He said: Then what (indeed)?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4534
Ibrahim bin Muhammad bin al-Muntashir narrates from his father that he was in the mosque of Amr bin Shurahbil. The iqamah for the prayer was called and the people were waiting for him. When he came, he said: "I was praying witr." The narrator said: Abdullah bin Mas'ud (may Allah be pleased with him) was asked: Is there witr after the adhan? He said: "Yes, and even after the iqamah." The narrator said: He narrated from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) slept through the prayer until the sun rose, then he woke up and prayed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4535