Uwaymir came to Asim bin Adi and said: "What do you think of a man who finds a (strange) man with his wife: he kills him, and then you kill him in retaliation? O Asim! Ask the Prophet (peace and blessings be upon him)." So Asim came and asked the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) disliked such questions and found fault with them. Asim returned and told Uwaymir that the Prophet (peace and blessings be upon him) had disliked such questions and found fault with them. Uwaymir said: "By Allah! I will certainly go to the Messenger of Allah (peace and blessings be upon him)." So he came, and by then the Qur'an had been revealed after Asim (had left). He asked the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) said: "Qur'an has been revealed concerning you." So he (peace and blessings be upon him) called the two of them; they came forward and performed li'an (mutual imprecation). Then Uwaymir said: "O Messenger of Allah! If I keep her, I will have lied against her." So he separated from her, although the Prophet (peace and blessings be upon him) had not ordered him to separate from her, and this became the practice of those who perform li'an. The Prophet (peace and blessings be upon him) said: "Watch: if she gives birth to a reddish, short child, like a gecko, then I think that he has lied against her; and if she gives birth to a dark child with black eyes and large buttocks, then I think that he has told the truth about her." So she gave birth to him in the disliked form.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5657