It is narrated from Sayyiduna Abu Sa'id (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever reaches the morning without having prayed Witr, then he has no Witr."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب ذكر الوتر وما فيه من السنن / 1092