Musnad Ahmad 658:90

Waki' narrated to us, Hasan ibn Salih narrated to us, from al-Suddi, from 'Adi ibn Thabit, from al-Bara', who said: I met my maternal uncle and he had the banner with him. I said: "Where do you intend to go?" He said: "The Messenger of Allah, may Allah bless him and grant him peace, sent me to a man who married his father's wife after him, to strike his neck, or kill him, and take his wealth."