It was narrated from Ibn 'Abbas that the Prophet (ﷺ) said:
"A believer should not be killed in retaliation for the murder of a disbeliever, and a person who has a treaty should not be killed during the time of the treaty." (TR)
English reference (TR): Vol. 3, Book 21, Hadith 2660 (TR)
Arabic reference (TR): Book 21, Hadith 2762 (TR)