Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Prophet (peace and blessings be upon him) came out, and signs of joy were apparent on his (peace and blessings be upon him) face. He (peace and blessings be upon him) said: "Indeed, Allah Almighty has granted you an additional prayer, and it is the witr."
Imam al-Bazzar and Imam al-Tabarani in al-Mu'jam al-Kabir recorded this narration; its chain contains a narrator, al-Nadr Abu Umar, who is extremely da'if (weak).