سنن النسائي

It was narrated that Abi Hassan said: "Ali said: 'The Messenger of Allah (ﷺ) did not tell me anything that he did not tell the people, except what is in a sheet in the sheath of my word.' They did not leave him alone until he brought out the sheet, and in it (were the words): 'The lives of the believers are equal in value, and they hasten to support the asylum granted by the least of them, and they are one against others. But no believer may be killed in return for a disbeliever, nor one with a covenant while his covenant is in effect."