Musannaf Ibn Abi Shayba 36:520

Yazid ibn Harun narrated to us, from Sufyan ibn Husayn, from al-Zuhri, from Ubayd Allah ibn Abd Allah ibn 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 be upon him) say: 'Whoever testifies that there is no god but Allah and that Muhammad is the Messenger of Allah, his wealth and his blood are inviolable, except by its right, and his reckoning is with Allah'?" Abu Bakr said, "I will not 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 with him, and by Allah, it was the right course." When he (Abu Bakr) was victorious over those of them he was victorious over, he said, "Choose between two options: either a devastating war or the disgraceful option." They said, "This devastating war, we have known it. So what is the disgraceful option?" 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.