سنن النسائي

It was narrated that Abu Hurairah said: "So Abu Bakr decided to fight them, then 'Umar said: 'O Abu Bakr, how can you fight the people when the Messenger of Allah (ﷺ) [SAW] said: "I have been commanded to fight the people until they say La ilaha illallah, and if they say it, their blood and their wealth will be safe from me except for a right that is due.?' Abu Bakr said: 'I will fight whoever separates prayer and Zakah. By Allah, if they withhold from me a young goat that they used to give to the Messenger of Allah (ﷺ) [SAW], I will fight them for withholding it.' 'Umar said: 'By Allah, as soon as I realized that Allah has expanded the chest of Abu Bakr to fight them, I knew that it was the truth.'"