Musnad Ahmad 27:560

Yazid narrated to us, he said: 'Abd al-Malik informed us, I heard Sa'id ibn Jubayr say: I asked Ibn 'Umar and said: "O Abu 'Abd al-Rahman, are the two who perform *li'an* (mutual cursing) separated?" He said: "Subhan Allah! Yes. Indeed, the first person to ask about that was so-and-so. He said: 'O Messenger of Allah, what do you think if one of us sees his wife committing an indecency? How should he act? If he remains silent, he remains silent upon a grave matter, and if he speaks, it is likewise.' The Messenger of Allah (ﷺ) remained silent and did not answer him. Then he stood up for his need. After that, he came to the Messenger of Allah (ﷺ) and said: 'Indeed, that which I asked you about, I have been afflicted with it.' He said: So Allah the Exalted revealed these verses in Surah al-Nur: {And those who accuse their wives...} [al-Nur: 6], until he completed the verses. He called the man, recited them to him, reminded him of Allah the Exalted, and informed him that the punishment of this world is lighter than the punishment of the Hereafter. He said: 'By the One who sent you with the truth, I did not lie about her.' Then he called the woman, exhorted her, reminded her, and informed her that the punishment of this world is lighter than the punishment of the Hereafter. She said: 'By the One who sent you with the truth, he is a liar.' He called the man, and he bore four testimonies by Allah that he was indeed among the truthful, and the fifth that the curse of Allah be upon him if he were among the liars. Then he called for the woman, and she bore four testimonies by Allah that he was indeed among the liars, and the fifth that the wrath of Allah be upon her if he were among the truthful. Then he separated them."