It is narrated from Sayyiduna al-Agharr al-Muzani (may Allah be pleased with him) that a man came to the Messenger of Allah (peace and blessings be upon him) and said: O Prophet of Allah, morning has come and I have not prayed witr. He (peace and blessings be upon him) said: "Witr is only at night." He said: O Prophet of Allah, morning has come and I have not prayed witr. He (peace and blessings be upon him) said: "Then pray witr now."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 887