Ja'far bin Muhammad narrates from his father that Sayyiduna Ali (may Allah be pleased with him) said concerning Ibn Muljam (after he had struck him): Feed him and give him drink, and keep him well as a captive. If I survive, I am the guardian of my own blood: if I wish I will pardon, and if I wish I will take retaliation. And if I do not survive and you kill him, do not mutilate him (muthlah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16759