Musannaf Ibn Abi Shayba 19:935

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from Simak, from Ibn 'Ubayd ibn al-Abras, from Ali ibn Abi Talib: That a man was brought to him who had been a Christian, then he accepted Islam, then he reverted to Christianity. He asked him about a word, and he said it to him. So Ali stood up to him and kicked him with his foot. Then the people stood up to him and struck him until they killed him.