(9, 10) باب الْقَوَدِ بَيْنَ الأَحْرَارِ وَالْمَمَالِيكِ فِي النَّفْسِ
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."
Reference: Sunan an-Nasa'i 4735
In-book reference: Book 45, Hadith 30
English translation: Vol. 5, Book 45, Hadith 4739