Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) said: Sayyiduna al-Harith bin Amr (may Allah be pleased with him) passed by me; the Messenger of Allah (peace and blessings be upon him) had tied a banner for him and sent him. I asked: Where has the Messenger of Allah (peace and blessings be upon him) sent you? He said: "He has sent me to a man who has married his father's wife after him, so that I may strike off his neck."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3392