Musnad Abi Ya'la 330 — he said: I found a scroll tied to the hilt

It is narrated from Abu Ja'far Muhammad bin Ali, from his father, from his grandfather, that he said: "I found a scroll tied to the hilt of the sword of the Messenger of Allah (peace and blessings be upon him), in which was written: 'Indeed, the most severe of people in hostility before Allah is the one who kills someone other than his killer, and the one who strikes someone other than the one who struck him; and whoever denies the favour of his mawali (the patrons who freed him) has indeed disowned what Allah Almighty sent down upon Muhammad (peace and blessings be upon him).'"