Musnad Ahmad 2:36

'Isam ibn Khalid and Abu al-Yaman narrated to us, they said: Shu'ayb ibn Abi Hamzah informed us, from al-Zuhri, who said: 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah ibn Mas'ud narrated to us: That Abu Hurayrah said: When the Messenger of Allah (peace be upon him) passed away, and Abu Bakr was after him, and those who disbelieved among the Arabs disbelieved, 'Umar said: "O Abu Bakr, how can you fight the people when the Messenger of Allah (peace be upon him) said: 'I have been commanded to fight the people until they say: "There is no god but Allah." Whoever says: "There is no god but Allah," his wealth and his life are protected from me, except by its right, and his reckoning is with Allah'?" Abu Bakr said: "By Allah, I will surely fight"—Abu al-Yaman said: "I will surely kill"—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 be upon him), I would fight them for withholding it." 'Umar said: "By Allah, it was nothing but that I saw that Allah, the Mighty and Majestic, had expanded the breast of Abu Bakr for the fighting, so I knew that it was the truth."