Mishkat al-Masabih 3475, 3476

‘Ali reported the Prophet (ﷺ) as saying, “The lives of all Muslims are equal; the lowliest of them can guarantee their protection, the most distant can keep others from breaking protection he has given,* and they are one band against others. A Muslim must not be killed for an infidel, nor must one who has been given a covenant be killed while his covenant holds.”
Abu Dawud and Nasa’i transmitted it, and Ibn Majah transmitted it on the authority of Ibn ‘Abbas.
* The interpretation which seems to be preferred is that no matter how far one may live from the land of infidels, any protection one may guarantee to an infidel must be respected by all Muslims. Another possible explanation is that when a detachment is sent ahead into infidel country those who remain behind them are entitled to their portion of any spoil they take, but it is felt that this meaning does not suit the context. The Arabic words are wa-yaruddu ‘alaihim aqsahum.