Musnad Ahmad 1237:6

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, Abdullah ibn Hubayrah informed us, he said: I heard Abu Tamim al-Jayshani say: I heard Amr ibn al-Aas say: A man from the companions of the Prophet, may Allah bless him and grant him peace, informed me, saying: The Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, Allah, the Almighty and Majestic, has added a prayer for you, so pray it between the Isha prayer and the morning prayer: the Witr, the Witr." And behold, it is Abu Basrah al-Ghifari. Abu Tamim said: Abu Dharr and I were sitting. He said: So Abu Dharr took my hand and we set off to Abu Basrah, and we found him at the door that adjoins the house of Amr ibn al-Aas. Abu Dharr said: "O Abu Basrah, did you hear the Prophet, may Allah bless him and grant him peace, say: 'Indeed, Allah, the Almighty and Majestic, has added a prayer for you, so pray it between the Isha prayer and the morning prayer: the Witr, the Witr'?" He said: "Yes." He said: "Did you hear it?" He said: "Yes." He said: "Did you hear it?" He said: "Yes."