Musannaf Ibn Abi Shayba 19:873

Abu Bakr narrated to us, he said: Yazid bin Harun narrated to us, he said: Sufyan bin Husayn informed us, from al-Zuhri, from Ubayd Allah bin Abd Allah bin Utbah, who said: When those who apostatized during the time of Abu Bakr apostatized, Abu Bakr wanted to fight them. Umar said: "Will you fight them when you have heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever testifies that there is no god but Allah and that Muhammad is the Messenger of Allah, his wealth and blood are inviolable, except by right, and his reckoning is with Allah the Exalted'?" Abu Bakr said: "By Allah, I will surely fight those who differentiate between prayer and zakat. By Allah, I will surely fight those who differentiate between them until I unite them." Umar said: "So we fought alongside him, and it was the right course." When he was victorious over those of them he was victorious over, he said: "Choose one of two traits from me: either the banishing war or the humiliating degradation." They said: "We have known this banishing war, so what is the humiliating degradation?" He said: "You testify regarding our dead that they are in Paradise, and regarding your dead that they are in the Fire." And they did so.