Sahih Bukhari Volume 006, Book 060, Hadith Number 135.
Narrated By Anas (bin Malik) : Ar-Rubai (the paternal aunt of Anas bin Malik) broke the incisor tooth of young Ansari girl. Her family demanded the Qisas and they came to the Prophet who passed the judgment of Qisas. Anas bin An-Nadr (the paternal uncle of Anas bin Malik) said, "O Allah's Apostle! By Allah, her tooth will not be broken." The Prophet said, "O Anas! (The law prescribed in) Allah's Book is Qisas." But the people (i.e. the relatives of the girl) gave up their claim and accepted a compensation. On that Allah's Apostle said, "Some of Allah's worshippers are such that if they take an oath, Allah will fulfill it for them."