Musnad Abu Ya'la al-Mawsili 117:245

Zuhayr narrated to us, Affan narrated to us, Hammad narrated to us, Thabit informed us, from Anas, that the sister of al-Rubayyi' ibn Harithah wounded a person, so they disputed before the Prophet (may Allah bless him and grant him peace). They said, "O Messenger of Allah, the retaliation, the retaliation." The mother of al-Rubayyi' said, "O Messenger of Allah, will you retaliate against so-and-so? No, by Allah, you shall not retaliate against her." She did not cease until they accepted the blood money. The Messenger of Allah (may Allah bless him and grant him peace) then said, "Indeed, among the servants of Allah are those who, if they were to swear by Allah, He would fulfill it for them."