Imam al-Shafi'i (may Allah have mercy on him) said: "Allah Almighty says: 'And whoever is killed unjustly, We have certainly given his heir authority (for retaliation)' [Surah al-Isra': 33].
And the Messenger of Allah (peace and blessings be upon him) said, concerning the things by which a Muslim's blood becomes lawful: 'and killing a soul without right.'
And it is narrated from the Messenger of Allah (peace and blessings be upon him): 'Whoever kills a Muslim without cause (unjustly), retaliation (shall be taken) for what his hand did.'"
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