Sahih Bukhari Volume 7, Book 63, Hadith Number 185.

Narrated By Sahl bin Sad As-Sa’idi : Uwaimir Al-‘Ajlani came to ‘Asim bin Adi Al-Ansari and asked, “O ‘Asim! Tell me, if a man sees his wife with another man, should he kill him, whereupon you would kill him in Qisas, or what should he do? O ‘Asim! Please ask Allah’s Apostle about that.” ‘Asim asked Allah’s Apostle about that. Allah’s Apostle disliked that question and considered it disgraceful. What ‘Asim heard from Allah’s Apostle was hard on him. When he returned to his family, ‘Uwaimir came to him and said “O ‘Asim! What did Allah’s Apostle say to you?” ‘Asim said, “You never bring me any good. Allah’s Apostle disliked to hear the problem which I asked him about.” ‘Uwaimir said, “By Allah, I will not leave the matter till I ask him about it.” So ‘Uwaimir proceeded till he came to Allah’s Apostle who was in the midst of the people and said, “O Allah’s Apostle! If a man finds with his wife another man, should he kill him, whereupon you would kill him (in Qisas): or otherwise, what should he do?” Allah’s Apostle said, “Allah has revealed something concerning the question of you and your wife. Go and bring her here.” So they both carried out the judgment of Lian, while I was present among the people (as a witness). When both of them had finished, ‘Uwaimir said, “O Allah’s Apostle! If I should now keep my wife with me, then I have told a lie”. Then he pronounced his decision to divorce her thrice before Allah’s Apostle ordered him to do so. (Ibn Shihab said, “That was the tradition for all those who are involved in a case of Lian.”