Musnad Ahmad 32:761

Muhammad ibn Abdullah ibn al-Muthanna narrated to us, Humayd al-Tawil narrated to us, from Anas ibn Malik, that al-Rubayyi’ bint al-Nadr, the paternal aunt of Anas ibn Malik, broke the front tooth of a young girl. They offered them the indemnity (arsh), but they refused and sought pardon, but they (the family of the girl) refused. So they came to the Prophet, peace and blessings of Allah be upon him, and he ordered retaliation (qisas). Then her brother, Anas ibn al-Nadr, the paternal uncle of Anas ibn Malik, came and said: "O Messenger of Allah, will the front tooth of al-Rubayyi’ be broken? No, by the One who sent you with the truth, her front tooth shall not be broken." The Messenger of Allah, peace and blessings of Allah be upon him, said: "O Anas, the Book of Allah is retaliation." He said: So the people pardoned. He said: And the Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, among the servants of Allah are those who, if they were to swear by Allah, He would fulfill it for them."