Musnad Ahmed: Hadith 3976

We were sitting in the mosque on the night before Friday, and a man among the Ansar said: If one of us sees a man with his wife and kills him, you will kill him, and if he speaks, you will flog him, but if he keeps quiet, he is keeping quiet when he is feeling frustrated and angry. By Allah, if I wake up safe and sound in the morning, I shall certainly ask The Messenger of Allah (ﷺ). So he asked him: O Messenger of Allah, if one of us sees a man with his wife and kills him, you will kill him, and if he speaks, you will flog him, but if he keeps quiet, he is keeping quiet when he is feeling frustrated and angry. O Allah, pass judgement. Then the verse of li’an was revealed, and that man was the first one to be tested with this experience.