Sahih Bukhari Volume 003, Book 049, Hadith Number 866.
Narated By Anas : Ar-Rabi, the daughter of An-Nadr broke the tooth of a girl, and the relatives of Ar-Rabi' requested the girl's relatives to accept the Irsh (compensation for wounds etc.) and forgive (the offender), but they refused. So, they went to the Prophet who ordered them to bring about retaliation. Anas bin An-Nadr asked, "O Allah"; Apostle! Will the tooth of Ar-Rabi' be broken? No, by Him Who has sent you with the Truth, her tooth will not be broken." The Prophet said, "O Anas! Allah"; law ordains retaliation." Later the relatives of the girl agreed and forgave her. The Prophet said, "There are some of Allah's slaves who, if they take an oath by Allah, are responded to by Allah i.e. their oath is fulfilled). Anas added, "The people agreed and accepted the Irsh."