Ikrimah (may Allah have mercy on him) said that when Ibn Abbas (may Allah be pleased with him) learned that Ali (may Allah be pleased with him) had burned the apostates and the heretics (zanadiqah), he said: Had it been me, I would not have burned them; rather I would have killed them, because the Prophet (peace and blessings be upon him) said: "Whoever changes his religion and apostatises, kill him." And I would not have burned them because the Prophet (peace and blessings be upon him) said: "It is not fitting for anyone to punish (anyone) with the punishment of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16820