Sahih Bukhari Volume 007, Book 063, Hadith Number 231.
Narrated By Said bin Jubair : I asked Ibn 'Umar, "(What is the verdict if) a man accuses his wife of illegal sexual intercourse?" Ibn 'Umar said, "The Prophet separated (by divorce) the couple of Bani Al-Ajlan, and said, (to them), 'Allah knows that one of you two is a liar; so will one of you repent?' But both of them refused. He again said, 'Allah knows that one of you two is a liar; so will one of you repent?' But both of them refused. So he separated them by divorce." (Aiyub, a sub-narrator said: 'Amr bin Dinar said to me, "There is something else in this Hadith which you have not mentioned. It goes thus: The man said, 'What about my money (i.e. the Mahr that I have given to my wife)?' It was said, 'You have no right to restore any money, for if you have spoken the truth (as regards the accusation), you have also consummated your marriage with her; and if you have told a lie, you are less rightful to have your money back.'")