Sahih Bukhari Volume 4, Book 52, Hadith Number 260.

Narated By Ikrima : Ali burnt some people and this news reached Ibn ‘Abbas, who said, “Had I been in his place I would not have burnt them, as the Prophet said, ‘Don’t punish (anybody) with Allah’s Punishment.’ No doubt, I would have killed them, for the Prophet said, ‘If somebody (a Muslim) discards his religion, kill him.'”