Musnad Ahmad 2:253

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, from Ma'mar, from al-Zuhri, from Ubayd Allah ibn Abd Allah ibn Utbah, from Abu Hurayrah, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) passed away, and those who disbelieved disbelieved, Umar ibn al-Khattab said: "O Abu Bakr, how can you fight the people 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." Whoever says: "There is no god but Allah," his wealth and his life are protected from me, and his reckoning is with Allah the Almighty and Majestic'?" Abu Bakr said: "I will surely fight whoever differentiates 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 chest of Abu Bakr for the fighting, so I knew that it was the truth."