Musannaf Ibn Abi Shayba 28:212

Ghundar narrated to us, from Shu'bah, from Simak, from Abu '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. Umar asked him about a word, and he said it to him. Then Ali stood up to him and kicked him with his foot. He said: Then the people stood up to him and struck him until they killed him.