Musannaf Ibn Abi Shayba 18:1157

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Abu ‘Asim, from al-Sha’bi, who said: There were two brothers from the Ansar, one of whom was called Ash’ath. He went out on a military expedition with one of the Muslim armies. The wife of his brother said to the brother: "Do you have any interest in your brother’s wife? There is a man with her, speaking to her." So he climbed up and looked down upon him while he was with her on her bed, and she was plucking a chicken for him. He was saying: "And Ash’ath, Islam has deceived him regarding me; I am alone with his wife for the full night. I spend the night on her cushions, while he spends the night On a dark, lean-flanked [camel] whose girth is tight. As if the places of her calves Are full moons gathered to my full moon." He said: The man leaped upon him and struck him with the sword until he killed him, then he threw him away. The next morning, he was found killed in Medina. ‘Umar said: "I adjure by Allah any man who has knowledge of this to stand up and speak." The man stood up and informed him of the story. He [‘Umar] said: "May he be crushed and cast far away."