Musnad Ahmad 658:112

Hushaym narrated to us, Ash'ath informed us, from 'Adi ibn Thabit, from al-Bara' ibn 'Azib, who said: My uncle, al-Harith ibn 'Amr, passed by me, and with him was a banner that the Prophet, may Allah bless him and grant him peace, had tied for him. I said to him, "O uncle, where did the Prophet, may Allah bless him and grant him peace, send you?" He said, "He sent me to a man who married his father's wife, and he ordered me to strike his neck."