Sunan Ibn Majah, Chapter: The Chapters on Blood Money

"Rubai, the paternal aunt of Anas, broke the tooth of a girl and they (her family) asked (the girl's family) to let her off, but they refused. They offered to pay compensatory money, but they refused. So they came to Prophet (saw) who ordered retaliation. Anas bin Nadr said: 'O Messenger of Allah (saw), will the tooth of Rubai' be broken? By the One Who sent you with the truth, it will not be broken!' The Prophet (saw) said: 'O Anas, what Allah has decreed is retaliation.' So the people accepted that and forgave her. The Messenger of Allah (saw) said: 'there are among the slaves of Allah those who, if they swear by Allah, Allah fulfills their oath.'"