Musnad Ahmad 658:141

Aswad ibn Amir narrated to us, Abu Bakr narrated to us, from Mutarrif, who said: "They came to a tent and brought out a man from it, then they killed him. He said: I said: 'What is this?' They said: 'This is a man who had intercourse with his wife's mother.' The Messenger of Allah, peace and blessings of Allah be upon him, had sent [someone] to him, so they killed him."