Musannaf Ibn Abi Shayba 28:625

'Abd al-Rahim ibn Sulayman narrated to us, from 'Abd al-Rahman ibn 'Ubayd, 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. They were brought to 'Ali ibn Abi Talib, and he 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.