سنن النسائي

It was narrated that Abu Hurairah said: "When Abu Bakr mobilized to fight them, 'Umar said: 'O Abu Bakr, how can you fight the people when the Messenger of Allah (ﷺ) said: "I have been commanded to fight the people until they say La ilaha illallah (there is none worthy or worship except Allah). Whoever says La ilaha illallah, his life and his property are safe from me, except for its right, and his reckoning will be with Allah?'" Abu Bakr, may Allah be pleased with me him, said: 'By Allah, I will surely 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 fight them for withholding it.' ('Umar said) 'By Allah, when I realized that Allah, the Most High, had opened the chest of Abu Bakr to fighting them, then I knew that it was the truth.'"