The Prophet ﷺ said, "Whoever kills a person bound by a covenant of protection from Allah and His Messenger has violated the covenant of Allah. Such a person will not smell the fragrance of Paradise, though its fragrance can be found from a distance of seventy years' travel."
عن النبي ﷺ قال: ألا من قتل نفسا معاهدة له ذمة الله، وذمة رسوله فقد أخفر بذمة الله، فلا يرح رائحة الجنة، وأن ريحها ليوجد من مسيرة سبعين خريفا.
Tirmidhi (1403) and Ibn Majah (2687) both narrated it from Muhammad bin Bashshar, who said, Madi bin Sulayman, who is Al-Basri, narrated to us, Ibn Ajlan told us, from his father, from Abu Hurairah, and he mentioned it.Tirmidhi said, "This is a hasan sahih hadith," and it has been narrated through other routes from Abu Hurairah, from the Prophet ﷺ.I say, its chain is hasan due to the discussion regarding Muhammad bin Ajlan, though he is one whose hadith is hasan.