Musnad Ahmad #231

It was narrated that `Ubaidullah bin `Abdullah bin `Utbah said: When the apostates apostatised at the time of Abu Bakr, ‘Umar said: How can you fight the people, O Abu Bakr, when the Messenger of Allah (ﷺ) said: “I have been commanded to fight the people until they say La ilaha illallah (There is no god but Allah), and whoever says La ilaha illallah, his wealth and his life are protected from me except in cases dictated by Islamic law, and his reckoning will be with Allah`? Abu Bakr said: By Allah, I will most certainly fight those who separate prayer and zakah, for zakah is what is due on wealth. By Allah, if they withhold from me a small she-goat that they used to give to the Messenger of Allah (ﷺ), I will certainly fight them for withholding it. `Umar bin al-Khattab said. By Allah, as soon as I saw that Allah had opened Abu Bakr`s heart to the idea of fighting, I knew that he was right.