"I heard Abu Juhaifah say: 'We asked 'Ali: "Do you have anything from the Messenger of Allah apart from the Qur'an?" He said: "No, by the One who splits the seeds and creates the soul, unless Allah gives a slave understanding of His Book, or except this sheet." I said: "What is in the sheet?" He said: "In it are (the regulations concerning) blood money and the freeing of captives and (the rule) that no Muslim should be killed for killing a disbeliever."
Sunan an-Nasa'i Book 45, Hadith 39
← Previous Hadith "It is not permissible to kill a Muslim except in one of three cases: A adulterer who has been married, who is to be stoned; a man who kills a Muslim deliberately; and a man who leaves Islam and wages war against Allah, the Mighty and Sublime, and His Messenger, who is to be killed, crucified or banished from the land."

Sunan an-Nasa'i Book 45, Hadith 38 Next Hadith → "Ali said: 'The Messenger of Allah 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."

Sunan an-Nasa'i Book 45, Hadith 40