Musnad Ahmad 32:362

Ibn Abi ‘Adi narrated to us, from Humayd, from Anas, that al-Rubayyi‘, the paternal aunt of Anas, broke the front tooth of a young girl. They requested the people to pardon her, but they refused. They came to the Messenger of Allah (peace and blessings of Allah be upon him), and he said: "Retaliation." Anas ibn al-Nadr said: "O Messenger of Allah, shall the front tooth of so-and-so 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: "He then said: 'By the One who sent you with the truth, the front tooth of so-and-so shall not be broken.'" He said: "The people became satisfied, so they pardoned and abandoned the retaliation." The Messenger of Allah (peace and blessings of Allah be upon him) then said: "Indeed, among the servants of Allah are those who, if they were to swear by Allah, He would fulfill it for them."