The Prophet (peace and blessings be upon him) said:
"A believer shall not be killed in retaliation for a disbeliever, nor shall one under a covenant (be killed) during the term of his covenant."
Hadith Referenceصحیح الجامع الصغیر / حرف اللام ألف / 7751
Hadith Gradingالألبانی:حسن
Hadith Takhrij[هـ] عن ابن عباس. الإرواء ٢٢٠٨، ٢٢٠٩.