Musannaf Ibn Abi Shayba 28:213

Abu al-Ahwas narrated to us, from Simak, from Qabus ibn al-Mukhariq, from his father, who said: Ali sent Muhammad ibn Abi Bakr as a governor over Egypt. He wrote to Ali asking him about the heretics (zanadiqa) among them, some of whom worship the sun and the moon, some of whom worship other things, and some of whom claim Islam. He wrote back to him and commanded him regarding the heretics to kill whoever claimed Islam, and to leave the rest of them to worship whatever they wished.