Musnad Ahmad 2:158

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah, who said: When the people of apostasy apostatized during the time of Abu Bakr, 'Umar said: "How can you fight the people, O Abu Bakr, when 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." So if they say: "There is no god but Allah," they have protected their blood and their wealth from me, except for its right, and their reckoning is with Allah'?" Abu Bakr said: "By Allah, I will surely fight those who differentiate between the prayer and the zakat, for the zakat is the right of wealth. By Allah, if they refused me a young she-goat that they used to pay to the Messenger of Allah (peace and blessings of Allah be upon him), I would fight them for it." 'Umar said: "By Allah, it was nothing but that I saw that Allah had expanded the breast of Abu Bakr for the fighting, so I knew that it was the truth."