Sahih Muslim Book 16, Hadith Number 4151.

Anas reported that Umm Haritha, the sister of Rubayyi’ (she was the father’s sister of Hadrat Anas) injured a person (she broke his teeth). The dispute was referred to Allah’s Apostle (may peace be upon him). Allah’s Messenger (may peace be upon him) said: Retribution, retribution. Umm Rubayyi’ said: Messenger of Allah, will retribution be taken from so and so? By Allah, it shall not be taken from her (i.e. from Umm Haritha). Thereupon Allah’s Apostle said: Hallowed be Allah. O Umm Rubayyi’, Qisas (retribution is a command, prescribed) in the Book of Allah. She said: No, by Allah, Qisas will never be taken from her; and she went on saying this until they (the relatives of the one who had been injured) accepted the blood-wit. Thereupon Allah’s Messenger (may peace be upon him) said: Verily there are amongst the servants of Allah (such pious persons) who, if they take oath of Allah, He honours it.