Hadith: Among Allah's slaves, there are some who, if they take an oath by Allah (concerning something), Allah fulfills it

عن أنس أن الرُّبَيِّعَ عمته كَسَرَتْ ثَنِيَّةَ جارية، فطلبوا إليها العفو فأبوا، فعرضوا الأرْشَ فأبَوْا، فأتوا رسول الله صلى الله عليه وسلم وأبوا إلا القصِاَصَ فأمر رسول الله صلى الله عليه وسلم بالقصاص، فقال أنس بن النضر: يا رسول الله أَتُكْسَرُ ثَنِيَّةُ الرُّبَيِّعِ؟ لا والذي بعثك بالحق لا تُكسر ثنيتها. فقال رسول الله صلى الله عليه وسلم : «يا أنسُ، كتابُ اللهِ القصاصُ». فرضي القومُ فَعَفَوْا، فقال رسول الله صلى الله عليه وسلم : «إن من عباد الله من لو أقسم على الله لأبَرَّهُ».
[صحيح] - [متفق عليه واللفظ للبخاري]

Anas reported that his aunt, Ar-Rubayyi‘, broke the incisor of a girl. The aunt's family requested the girl's relatives for pardon, but they refused, then they proposed a compensation, but they refused. Then they went to the Messenger of Allah (may Allah's peace and blessings be upon him) and refused everything except retribution. So the Messenger of Allah (may Allah's peace and blessings be upon him) judged that retribution be applied. Anas ibn An-Nadr said: "O Messenger of Allah, will the incisor of Ar-Rubayyi‘ be broken? No, by the One Who sent you with the Truth, her incisor will not be broken." Therefore, the Messenger of Allah (may Allah's peace and blessings be upon him) said: "O Anas, the prescribed law of Allah is retribution." Thereupon, those people agreed on pardon. Then the Messenger of Allah (may Allah's peace and blessings be upon him) said: "Among Allah's slaves, there are some who, if they take an oath by Allah (concerning something), Allah fulfills it."
Sahih/Authentic. - [Al-Bukhari and Muslim. This is the wording of Al-Bukhari]

Explanation

This Hadīth indicates that Ar-Rubayyi‘ (may Allah be pleased with her) broke some of the front teeth of a girl from the Ansār. So the Prophet (may Allah's peace and blessings be upon him) wanted that retribution be inflicted upon her, such that her equivalent front teeth be broken. However, her brother Anas ibn An-Nadr asked rhetorically, not denying the judgment of Allah, whether her teeth should be broken. He swore, out of good faith in Allah, that her teeth would not be broken. Nevertheless, the Prophet (may Allah's peace and blessings be upon him) reminded him that the divine judgment entails retribution. So when the victim's family saw that, they agreed to accept the compensation and gave up retribution. Hence, the Prophet (may Allah's peace and blessings be upon him) said that among the slaves of Allah, there are some people who, if they swear an oath, Allah fulfills it for them, owing to their righteousness and trust in Allah.