Musnad Ahmad 1013:34

Abu Kamil narrated to us, Ibrahim—meaning Ibn Sa'd—narrated to us, Ibn Shihab narrated to us, from Sahl ibn Sa'd, who said: 'Uwaymir came to 'Asim ibn 'Adi and said: "Ask the Messenger of Allah (may Allah bless him and grant him peace): What do you think of a man who finds a man with his wife and kills him; is he to be killed for it, or how should he act?" 'Asim asked the Messenger of Allah (may Allah bless him and grant him peace), and the Messenger of Allah (may Allah bless him and grant him peace) disapproved of the questions. 'Uwaymir met him and said: "What did you do?" He replied: "You did not bring me any good. I asked the Messenger of Allah (may Allah bless him and grant him peace), and he disapproved of the questions." 'Uwaymir said: "By Allah, I will certainly go to the Messenger of Allah (may Allah bless him and grant him peace) and ask him." He came to him and found that [the verses] had been revealed concerning them. He called for them both and performed the *li'an* (oath of imprecation) between them. 'Uwaymir said: "If I keep her, O Messenger of Allah, I have lied against her." So he separated from her before the Messenger of Allah (may Allah bless him and grant him peace) ordered him to. It became the *sunnah* (practice) for those who perform *li'an*. The Messenger of Allah (may Allah bless him and grant him peace) said: "Observe her; if she gives birth to him [the child] dark-skinned, with deep black eyes, and large buttocks, then I do not see him but as having told the truth. And if she gives birth to him red-skinned, as if he were a *wahrah* (a type of lizard), then I do not see him but as a liar." She gave birth to him according to the disliked description.