Sayyiduna al-Agharr al-Muzani (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "Whoever is overtaken by the morning without having prayed the Witr, there is no Witr for him any longer."
Al-Bazzar recorded it from Salih bin Mu'adh al-Baghdadi, who is his teacher; I do not know him. The rest of its narrators are thiqat (trustworthy).