Musnad Ahmad 26:451

Yahya ibn Hammad narrated to us, Abu 'Awanah narrated to us, from al-A'mash, from Ibrahim, from 'Alqamah, from 'Abdullah, who said: We were sitting on the eve of Friday in the mosque. A man from the Ansar said: "One of us saw a man with his wife and killed him; you killed him [in retaliation]. If he speaks, you flog him; and if he remains silent, he remains silent upon rage. By Allah, if I reach the morning in a sound state, I will surely ask the Messenger of Allah (ﷺ)." He said: So he asked him and said: "O Messenger of Allah, one of us saw a man with his wife and killed him; you killed him [in retaliation]. If he speaks, you flog him; and if he remains silent, he remains silent upon rage. O Allah, judge [between us]." He said: "Then the verse of Li'an (imprecation) was revealed." He said: And that man was the first to be tested by it.