It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that his paternal aunt, Sayyidah Rubayyi' bint al-Nadr (may Allah be pleased with her), slapped a slave girl and broke her tooth. They offered (the slave girl's family) compensation, but they refused; then they asked for forgiveness, but they (also) refused. So they came to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) ordered retaliation (qisas). Thereupon her brother, Sayyiduna Anas bin al-Nadr (may Allah be pleased with him), came and said: O Messenger of Allah, will the tooth of Rubayyi' be broken? No! By the One who sent you with the truth, her tooth will not be broken. He (peace and blessings be upon him) said: "O Anas, the Book of Allah prescribes retaliation." Thereupon the people forgave, and the Messenger of Allah (peace and blessings be upon him) said: "Among the servants of Allah are those who, if they swear by Allah, Allah fulfils their oath."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 764