Hadith: The lives of all Muslims are equal; they are one hand against the others. The covenant (given by even the) lowliest of them must be respected. Behold, a Muslim must not be killed for a disbeliever, nor must one who has been given a covenant be killed while his covenant is still in force. If anyone introduces an evil, he will be responsible for it. Whoever introduces an evil deed or shelters a perpetrator incurs the curse of Allah, His Angels, and all the people

عن قيس بن عباد، قال: انطلقت أنا والأشتر، إلى علي رضي الله عنه فقلنا: هل عَهِدَ إليك رسول الله صلى الله عليه وسلم شيئًا لم يَعْهَدْهُ إلى الناس عامة؟ قال: لا، إلا ما في كتابي هذا، قال مسدد: قال: فأخرج كتابًا، وقال أحمد: كتابا من قِرَابِ سيفه، فإذا فيه «المؤمنون تَكَافَأُ دماؤهم، وهم يد على من سِوَاهم، ويسعى بذِّمَّتِهِم أدناهم، ألا لا يُقتل مؤمن بكافر، ولا ذُو عَهْد في عهده، من أحدث حَدَثَاً فعلى نفسه، ومن أحدث حدثا، أو آوى مُحْدِثاً فعليه لعنة الله والملائكة والناس أجمعين».
[صحيح] - [رواه أبو داود والنسائي وأحمد]

Qays ibn ‘Abbād reported: Al-Ashtar and I went to ‘Ali (may Allah be pleased with him) and said to him: "Has the Messenger of Allah (may Allah's peace and blessings be upon him) given you anything exclusive that he did not give to the people in general?" He said: "No, except what is in this document of mine." Mūsaddad said: "He then took out a document." Ahmad said: "A document from the sheath of his sword." It stipulated that "the lives of all Muslims are equal; they are one hand against the others. The covenant (given by even the) lowliest of them must be respected. Behold, a Muslim must not be killed for a disbeliever, nor must one who has been given a covenant be killed while his covenant is still in force. If anyone introduces an evil, he will be responsible for it. Whoever introduces an evil deed or shelters a perpetrator incurs the curse of Allah, His Angels, and all the people."
[Authentic hadith] - [An-Nasaa’i - Narrated by Abu Daoud - Narrated by Ahmad]

Explanation

This Hadīth indicates that some of the Tābi‘īn asked ‘Ali (may Allah be pleased with him) a question similar to that asked by Abu Juhayfah (may Allah be pleased with him) on another occasion. He asked if the Messenger of Allah (may Allah's peace and blessings be upon him) had given ‘Ali any exclusive knowledge different from the Qur’an? ‘Ali (may Allah be pleased with him) answered in the negative, affirming that he had nothing but a sheet on which were some rulings which ‘Ali had written from the Messenger of Allah (may Allah's peace and blessings be upon him). These included the ruling that the lives of all Muslims are equal, such that a Muslim would be killed for killing any Muslim. However, a Muslim would not be killed for killing a disbeliever, for a disbeliever is not equal to a Muslim in the inviolability of their blood. Also, the covenant of any Muslim is to be respected, whether it is given by a junior or a senior person, by a man or a woman. So if a Muslim gives a covenant of protection to someone, even if the person who is to be protected is a disbeliever, the covenant must be honored. It is not permissible to kill anyone who enters the lands of Islam under a covenant, for his life has been protected by virtue of this covenant. Anyone who does an evil deed or hides or shelters the perpetrator incurs the curse of Allah, the Angels, and all people. This means that he shall be dismissed from the mercy of Allah, the Exalted.