Musnad Ahmad 1013:59

Hashim narrated to us, Abd al-Aziz—meaning Ibn Abi Salamah—narrated to us, from al-Zuhri, from Sahl ibn Sa’d al-Sa’idi, from ‘Asim ibn ‘Adi, who said: ‘Uwaymir, a man from Banu ‘Ajlan, came to him and said: "O ‘Asim, what do you think of a man who finds a man with his wife? Should he kill him, so they kill him? Or how should he act? Ask the Messenger of Allah for me, O ‘Asim." So ‘Asim asked the Messenger of Allah (peace and blessings of Allah be upon him) about that. The Messenger of Allah (peace and blessings of Allah be upon him) disliked such questions and criticized them, until it became burdensome for ‘Asim to hear what he heard from the Messenger of Allah (peace and blessings of Allah be upon him). He then mentioned the meaning of Malik’s hadith, except that he said: "So he divorced her before the Prophet (peace and blessings of Allah be upon him) commanded him to." He said: "So his separation from her became the Sunnah for those who engage in mutual imprecation (li’an)."