Uwaymir, a man of the Ansar from Banu al-Ajlan, came to Asim bin Adi and said: "Ask the Prophet (peace and blessings be upon him): what do you think of a man who finds a (strange) man with his wife and kills him: will you kill him, or what should he do?" The man (Uwaymir) waited for a while, then came and said: "O Asim! What did you do about what I said?" Asim said: "You have never brought me anything good. I asked the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) found fault with such questions." His heart was not at rest, until he came to the Messenger of Allah (peace and blessings be upon him) himself and said to him (peace and blessings be upon him) the same thing he had said to Asim. He (peace and blessings be upon him) said: "Come, Allah, the Mighty and Majestic, has revealed a verse concerning you and your companion." So the Messenger of Allah (peace and blessings be upon him) called the two of them and had them perform li'an (mutual imprecation). Uwaymir said: "O Prophet of Allah! If I keep her after performing li'an with her, I will have lied against her." So the practice of separating the couple who perform li'an became established.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5665