Musnad Ahmad 27:246

Yahya ibn Sa'id narrated to us, 'Abd al-Malik ibn Abi Sulayman narrated to us, I heard Sa'id ibn Jubayr say: I was asked about the two who perform *li'an* (mutual imprecation): Is there a separation between them? This was during the governorship of Ibn al-Zubayr, and I did not know what to say. So I stood from my place and went to the house of Ibn 'Umar and said: "O Abu 'Abd al-Rahman, the two who perform *li'an*, is there a separation between them?" He said: "Subhan Allah! The first person to ask about that was such-and-such son of such-and-such. He said: 'O Messenger of Allah, what if a man sees his wife committing an indecency? If he speaks, he speaks of a grave matter, and if he remains silent, he remains silent upon the like of that.' The Prophet remained silent and did not answer him. Then, after a while, he came to him and said: 'The matter I asked you about, I have been afflicted with it.' Then Allah, the Mighty and Majestic, revealed these verses in Surah al-Nur: 'And those who accuse their wives...' until He reached: '...that the wrath of Allah be upon her if he is of the truthful.' The Prophet began with the man, admonished him, reminded him, 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 have not lied against her.' Then he turned to the woman, admonished 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 said: 'He began with the man, who testified four times by Allah that he was indeed of the truthful, and the fifth time that the curse of Allah be upon him if he were of the liars. Then he turned to the woman, who testified four times by Allah that he was indeed of the liars, and the fifth time that the wrath of Allah be upon her if he were of the truthful.' Then he separated them."