Musnad Ahmad 26:731

'Abd al-Rahman ibn Muhammad al-Muharibi narrated to us, from al-A'mash, from Ibrahim. 'Abdullah ibn Ahmad said: My father said: And another [narrator] said, from 'Alqama, who said: 'Abdullah said: While we were in the mosque on the night of Friday, a man from the Ansar said: "By Allah, if a man finds a man with his wife and speaks, he will be flogged; and if he kills him, he will be killed; and if he remains silent, he will remain silent upon rage. By Allah, when morning comes, I will certainly go to the Messenger of Allah (peace and blessings of Allah be upon him)." When morning came, he went to the Messenger of Allah (peace and blessings of Allah be upon him) and said: "O Messenger of Allah, if a man finds a man with his wife and speaks, he will be flogged; and if he kills him, he will be killed; and if he remains silent, he will remain silent upon rage?" And he began to say: "O Allah, grant a way out! O Allah, grant a way out!" He said: Then the verses of *Li'an* (imprecation) were revealed: "And those who accuse their wives and have no witnesses except themselves..." [Surah an-Nur: 6].