Musannaf Ibn Abi Shayba 19:933

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Simak, from Qabus ibn Mukhariq, from his father, who said: Ali sent Muhammad ibn Abi Bakr as a governor over Egypt. Muhammad 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 than that, and some of whom claim Islam. Ali wrote back and ordered regarding the heretics that whoever claims Islam should be killed, and the rest of them should be left to worship whatever they wish.