كتاب الفضائل
Read the English translation first, with the original Arabic directly underneath.
Abu Hurairah (May Allah be pleased with him) reported:
When the Messenger of Allah (ﷺ) passed away, Abu Bakr (May Allah be pleased with him) was appointed as his successor (caliph). Amongst the Arabs some men apostatised. Abu Bakr (May Allah be pleased with him) resolved to fight them. 'Umar bin Al-Khattab (May Allah be pleased with him) said to Abu Bakr: "How can you fight them when the Messenger of Allah (ﷺ) has declared: 'I have been commanded to fight people till they testify La ilaha illallah (there is no true god except Allah); and if they do it, their blood (life) and property are secured except when justified by law, and it is for Allah to call them to account." Upon this Abu Bakr (May Allah be pleased with him) said: "By Allah, I would definitely fight him who makes distinction between Salat and the Zakat, because it is an obligation upon the rich to pay Zakat. By Allah I will fight them even to secure the piece of rope which they used to give to the Messenger of Allah (ﷺ)." 'Umar (May Allah be pleased with him) said: "I realized that Allah opened the heart of Abu Bakr (May Allah be pleased with him) for fighting those who refused to pay Zakat, and I fully recognized that Abu Bakr
أمرت أن أقاتل الناس حتى يقولوا لا إله إلا الله، فمن قالها، فقد عصم مني ماله ونفسه إلا بحقه وحسابه على الله