Musnad Ahmad 1013:56

Hajjaj narrated to us, Laith ibn Sa’d narrated to us, ‘Aqil ibn Khalid narrated to me, from Ibn Shihab, from Sahl, that he said: A man from the Ansar came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, have you seen a man who finds a man with his wife; should he kill him?" He said: So Allah, the Mighty and Majestic, revealed regarding his affair what was mentioned in the Quran concerning the mutual cursing (li’an), and he said: "It has been decreed regarding you and your wife." He said: So they performed the mutual cursing while I was a witness, then he separated from her in the presence of the Messenger of Allah, peace and blessings of Allah be upon him.