Musnad Ahmad 788:207

Abu Abd al-Rahman Mu’ammal narrated to us, he said: Hammad—meaning Ibn Salamah—narrated to us, he said: ‘Asim narrated to us, from Abu Wa’il, from Abu Musa, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Allah, place ‘Ubayd Abu ‘Amir above most people on the Day of Resurrection." He said: So ‘Ubayd was killed on the day of Awtas, and Abu Musa killed the killer of ‘Ubayd. He said: Abu Wa’il said: I hope that Allah (Mighty and Majestic is He) will not bring together the killer of ‘Ubayd and Abu Musa in the Fire.