Sayyiduna Anas (may Allah be pleased with him) said: Sayyidah al-Rubayyi' bint al-Nadr (may Allah be pleased with her) slapped a slave girl and broke her front teeth. They asked for pardon, but they (the girl's people) refused; they offered compensation, and they refused again. The matter came before the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) ordered retaliation. Sayyiduna Anas bin al-Nadr (may Allah be pleased with him) said: O Messenger of Allah (peace and blessings be upon him)! Will the front teeth of al-Rubayyi' also be broken? By the One who sent you with the truth, that cannot happen. He (peace and blessings be upon him) said: "O Anas! The ruling of the Book of Allah is retaliation." The people were satisfied and pardoned (her). Then he (peace and blessings be upon him) said: "Among the servants of Allah are some who, if they swear an oath by Allah, Allah fulfils their oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16096