Musnad Ahmad 1107:4

Ali ibn Ishaq narrated to us, Abdullah—meaning Ibn al-Mubarak—narrated to us, Sa'id ibn Yazid informed us, Ibn Hubayrah narrated to me, from Abu Tamim al-Jayshani, that Amr ibn al-Aas addressed the people on a Friday and said: "Indeed, Abu Basrah narrated to me that the Prophet, peace and blessings of Allah be upon him, said: 'Verily, Allah has added a prayer for you, and it is the Witr, so pray it between the Isha prayer and the dawn prayer.'" Abu Tamim said: Then Abu Dharr took my hand and walked in the mosque to Abu Basrah and said to him: "Did you hear the Messenger of Allah, peace and blessings of Allah be upon him, say what Amr said?" Abu Basrah said: "I heard it from the Messenger of Allah, peace and blessings of Allah be upon him."