Sufyan says: I saw Amr bin Dinar, Ayyub and Ammar al-Duhni gathered together, and they discussed those whom Ali (may Allah be pleased with him) had burned.
(b) Ikrimah narrates from Abdullah bin Abbas (may Allah be pleased with them both): Had it been me, I would not have burned them, because of the saying of the Messenger of Allah (peace and blessings be upon him): "Do not punish with the punishment of Allah," and I would have killed them because of the saying of the Messenger of Allah (peace and blessings be upon him): "Whoever apostatises (changes his religion), kill him." Ammar says: They were not burned; rather, pits were dug for them, with holes made (between them) inside, then smoke was let in upon them until they died. Amr said: The poet has said: Cast me before death however you wish, so long as you have not cast me into the two pits when they have gathered firewood and fire; then death comes at once and does not delay.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18062