Sahih Muslim 20
It is narrated on the authority of Abu Huraira that when the Messenger of Allah (ﷺ) breathed his last and Abu Bakr was appointed as his successor (Caliph), those amongst the Arabs who wanted to become apostates became apostates. 'Umar b. Khattab said to Abu Bakr:
Why would you fight against the people, when the Messenger of Allah declared: I have been directed to fight against people so long as they do not say: There is no god but Allah, and he who professed it was granted full protection of his property and life on my behalf except for a right? His (other) affairs rest with Allah. Upon this Abu Bakr said: By Allah, I would definitely fight against him who severed prayer from Zakat, for it is the obligation upon the rich. By Allah, I would fight against them even to secure the cord (used for hobbling the feet of a camel) which they used to give to the Messenger of Allah (as zakat) but now they have withheld it. Umar b. Khattab remarked: By Allah, I found nothing but the fact that Allah had opened the heart of Abu Bakr for (perceiving the justification of) fighting (against those who refused to pay Zakat) and I fully recognized that the (stand of Abu Bakr) was right.
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا لَيْثُ بْنُ سَعْدٍ، عَنْ عُقَيْلٍ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ بْنِ مَسْعُودٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَاسْتُخْلِفَ أَبُو بَكْرٍ بَعْدَهُ وَكَفَرَ مَنْ كَفَرَ مِنَ الْعَرَبِ قَالَ عُمَرُ بْنُ الْخَطَّابِ لأَبِي بَكْرٍ كَيْفَ تُقَاتِلُ النَّاسَ وَقَدْ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَقُولُوا لاَ إِلَهَ إِلاَّ اللَّهُ فَمَنْ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ فَقَدْ عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ إِلاَّ بِحَقِّهِ وَحِسَابُهُ عَلَى اللَّهِ ‏"‏ ‏.‏ فَقَالَ أَبُو بَكْرٍ وَاللَّهِ لأُقَاتِلَنَّ مَنْ فَرَّقَ بَيْنَ الصَّلاَةِ وَالزَّكَاةِ فَإِنَّ الزَّكَاةَ حَقُّ الْمَالِ وَاللَّهِ لَوْ مَنَعُونِي عِقَالاً كَانُوا يُؤَدُّونَهُ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم لَقَاتَلْتُهُمْ عَلَى مَنْعِهِ ‏.‏ فَقَالَ عُمَرُ بْنُ الْخَطَّابِ فَوَاللَّهِ مَا هُوَ إِلاَّ أَنْ رَأَيْتُ اللَّهَ عَزَّ وَجَلَّ قَدْ شَرَحَ صَدْرَ أَبِي بَكْرٍ لِلْقِتَالِ فَعَرَفْتُ أَنَّهُ الْحَقُّ ‏.‏