سنن النسائي

It was narrated that Qais bin 'Ubad said: "Al-Ashtar and I went to 'Ali, may Allah be pleased with him, and said: Did the Prophet (ﷺ) of Allah tell you anything that he did not tell to all the people?' He said: 'No, except what is in this letter of mine.' He brought out a letter from the sheath of his sword and it said therein: "The lives of the believers are equal in value, and they are one against others, and they hasten to support the asylum granted by the least of them. But no believer may be killed in return for a disbeliever, nor one with a covenant while his convenant is in effect. Whoever commits an offense then the blame is on himself, and whoever gives sanctuary to an offender, then upon him will be the curse of Allah, the angels and all the people."