سنن ابن ماجه

It was narrated that Bara bin Azib said: “My maternal uncle passed by me - (one of the narrators) Hushaim named him in his narration as Harith bin Amr - and the Prophet (ﷺ) had given him a banner to carry. I said to him: 'Where are you going?’ He said: 'The Messenger of Allah (ﷺ) has sent me to a man who married his father's wife after he died, and has commanded me to strike his neck (i.e. execute him).”