Hadith: Whoever kills a mu‘āhad (a non-Muslim who is granted the pledge of protection by the Muslims) shall not smell the fragrance of Paradise, though its fragrance can be smelt at a distance of forty years

عن عبد الله بن عمرو رضي الله عنهما عن النبي صلى الله عليه وسلم قال:
«مَنْ قَتَلَ مُعَاهَدًا ‌لَمْ ‌يَرَحْ ‌رَائِحَةَ الْجَنَّةِ، وَإِنَّ رِيحَهَا تُوجَدُ مِنْ مَسِيرَةِ أَرْبَعِينَ عَامًا».
[صحيح] - [رواه البخاري] - [صحيح البخاري: 3166]

‘Abdullāh ibn ‘Amr (may Allah be pleased with him) reported: The Prophet (may Allah’s peace and blessings be upon him) said:
"Whoever kills a mu‘āhad (a non-Muslim who is granted the pledge of protection by the Muslims) shall not smell the fragrance of Paradise, though its fragrance can be smelt at a distance of forty years."
[Authentic] - [Narrated by Al-Bukhāri] - [Sahīh Al-Bukhāri - 3166]

Explanation

The Prophet (may Allah's peace and blessings be upon him) demonstrates the stern warning to anyone who kills a mu‘āhad - a non-Muslim who comes from the land of disbelievers to the abode of Islam and receives the pledge of protection and security - that he will not smell the fragrance of Paradise, though its fragrance can be smelt at a distance of forty years journey.

Benefits from the Hadith

1. It is prohibited to kill a mu‘āhad, a dhimmi, or a musta’man. This is a major sin.
2. Mu‘āhad: He is a disbeliever with whom a covenant was made while he lives in his land and does not fight the Muslims, nor do they fight him. Dhimmi: He is a non-Muslim who comes to live in the Muslim land and pays tribute. Musta’man: He is a non-Muslim who enters the land of the Muslims under a covenant and pledge of protection for a certain period of time.
3. It warns against the betrayal of covenants with non-Muslims.