Musannaf Ibn Abi Shayba 28:217

'Abd al-Rahim ibn Sulayman narrated to us, from 'Abd al-Rahman ibn 'Ubayd al-'Amiri, from his father, who said: There were people who used to take the stipend and the provision and pray with the people, while they were worshipping idols in secret. So 'Ali ibn Abi Talib brought them and placed them in the mosque—or he said: in the prison. Then he said: "O people, what is your opinion regarding a people who used to take the stipend and the provision with you, while they were worshipping these idols?" The people said: "Kill them." He said: "No, but I will do to them as was done to our father Abraham." So he burned them with fire.