When the Messenger of Allah ﷺ died and Abu Bakr was appointed as his successor, some of the Arabs reverted to disbelief.Umar bin Khattab said to Abu Bakr, "How can you fight these people when the Messenger of Allah ﷺ said, 'I have been commanded to fight people until they say there is no god but Allah. Whoever says there is no god but Allah has protected his wealth and life from me, except by its right, and his reckoning is with Allah'?"Abu Bakr replied, "By Allah, I will fight anyone who separates prayer from zakat, for zakat is the right due upon wealth. By Allah, if they withhold from me even a hobbling rope that they used to give to the Messenger of Allah ﷺ, I will fight them for withholding it."Umar bin Khattab said, "By Allah, when I saw that Allah had opened Abu Bakr's heart to fighting them, I knew it was the truth."
حدثنا قتيبة حدثنا الليث عن عقيل عن الزهري أخبرني عبيد الله بن عبد الله بن عتبة بن مسعود
لما توفي رسول الله ﷺ واستخلف أبو بكر بعده كفر من كفر من العرب فقال عمر بن الخطاب لأبي بكر كيف تقاتل الناس وقد قال رسول الله ﷺ أمرت أن أقاتل الناس حتى يقولوا لا إله إلا الله ومن قال لا إله إلا الله عصم مني ماله ونفسه إلا بحقه وحسابه على الله قال أبو بكر والله لأقاتلن من فرق بين الزكاة والصلاة وإن الزكاة حق المال والله لو منعوني عقالا كانوا يؤدونه إلى رسول الله ﷺ لقاتلتهم على منعه فقال عمر بن الخطاب فوالله ما هو إلا أن رأيت أن الله قد شرح صدر أبي بكر للقتال فعرفت أنه الحق
Sourced. Agreed upon [by Bukhari and Muslim]. Bukhari narrated it in "Holding Fast to the Book and the Sunnah," the chapter on following the practices of the Messenger of Allah, hadith no. 7284 and 7285, and Muslim narrated it, hadith no. 20, both from Qutayba, with this wording.