Musnad Abu Ya'la al-Mawsili 152:93

On the authority of Abdullah, he said: I was in the mosque on the night of Jumu'ah when a man from the Ansar entered and said: "If a man were to find a man with his wife, and he spoke, you would flog him; and if he killed him, you would kill him; and if he remained silent, he would remain silent upon rage. By Allah, I will certainly ask the Messenger of Allah (ﷺ) about this." When the next day came, he went to the Messenger of Allah (ﷺ) and asked him, saying: "If a man were to find a man with his wife, and he spoke, you would flog him; or if he killed him, you would kill him; or if he remained silent, he would remain silent upon rage." He (the Prophet) said: "O Allah, grant a way out," and he began to supplicate. Then the verse of Li'an (imprecation) was revealed: "And those who accuse their wives and have no witnesses except themselves..." [Surah An-Nur: 6]—these verses. The man was tested with this among the people. He and his wife came to the Messenger of Allah (ﷺ), and they performed the Li'an. The man testified four times by Allah that he was indeed truthful, then he invoked the fifth time that the curse of Allah be upon him if he were of the liars. She then went to perform the imprecation, and the Messenger of Allah (ﷺ) said: "Wait." But she performed the imprecation. When they turned to leave, he (the Prophet) said: "It is likely that she will bring forth a child who is black and curly-haired." And she did indeed bring forth a child who was black and curly-haired.