Beheading one who marries his father's wife

It is narrated from Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) that he said: I met my maternal uncle (Sayyiduna Abu Burdah bin Niyar, may Allah be pleased with him) while he was carrying a banner. I asked: "Where do you intend to go?" He said: "The Prophet (peace and blessings be upon him) has sent me to a man who, after his father's death, married his father's wife (his stepmother), and he (peace and blessings be upon him) has commanded me to strike off his head."
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it. It also has other supporting reports from Adi bin Thabit and from al-Bara'.

It is narrated from Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) that some people passed by us who were setting out (on a mission). We asked them: "Where are you going?" They said: "The Messenger of Allah (peace and blessings be upon him) has sent us to a man who has married his father's wife, so that we may kill him."

It is narrated from Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) that he said: In the time of the Messenger of Allah (peace and blessings be upon him), I was searching for some camels of mine that had gone astray. While I was wandering among some houses, I suddenly saw riders and horsemen who came and surrounded (a house) and brought a man out. They neither questioned him nor spoke to him, until they struck off his head. When they had gone, I asked about him, and the people said: "He had married his father's wife."