Musnad Ahmad 30:3709

Rawh narrated to us, Muhammad ibn Abi Hafsah narrated to us, al-Zuhri narrated to us, from Ubayd Allah ibn Abd Allah, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have been commanded to fight the people until they say: 'There is no god but Allah.' If they say it, their blood and their wealth are protected from me, and their reckoning is with Allah." He said: When Abu Bakr stood up and those who apostatized apostatized, Abu Bakr wanted to fight them. Umar said: "How can you fight these people when they pray?" Abu Bakr said: "By Allah, I will surely fight those who have apostatized regarding Zakat. By Allah, if they withheld from me a young she-goat that they used to pay to the Messenger of Allah, I would fight them for it." Umar said: "When I saw that Allah had expanded the breast of Abu Bakr for fighting them, I knew that it was the truth."