Chapter 45: The Book of Oaths (qasamah), Retaliation and Blood Money
It was narrated from 'Ali, may Allah be please with him that the Prophet said: "The lives of the believers are equal in value, and they are one against others, and they hasten to support the asylum granted by the least of them. But no believer may be killed in return for a disbeliever, nor one with a covenant while his covenant is in effect."
Sunan an-Nasa'i 4735