Narrated Abu Huraira:

When Allah's Apostle died and Abu Bakr was elected as a Caliph after him, some of the Arabs reverted to disbelief, 'Umar said to Abu Bakr, "How dare you fight the people while Allah's Apostle ﷺ said, I have been ordered to fight the people till they say 'None has the right to be worshipped but Allah' And whoever says: None has the right to be worshipped but Allah.' waves his wealth and his life from me unless he deserves a legal punishment lusty, and his account will be with Allah! Abu Bakr said, "By Allah, I will fight him who discriminates between Zakat and prayers, for Zakat is the Compulsory right to be taken from the wealth By Allah, if they refuse to give me even a tying rope which they use to give to Allah's Apostle, I would fight them for withholding it." 'Umar said, 'By Allah, It was nothing, except I saw that Allah had opened the chest of Abu Bakr to the fight, and I came to know for certain that was the truth."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا لَيْثٌ، عَنْ عُقَيْلٍ، عَنِ الزُّهْرِيِّ، أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللَّهِ ﷺ وَاسْتُخْلِفَ أَبُو بَكْرٍ بَعْدَهُ، وَكَفَرَ مَنْ كَفَرَ مِنَ الْعَرَبِ قَالَ عُمَرُ لأَبِي بَكْرٍ كَيْفَ تُقَاتِلُ النَّاسَ، وَقَدْ قَالَ رَسُولُ اللَّهِ ﷺ
" أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَقُولُوا لاَ إِلَهَ إِلاَّ اللَّهُ. فَمَنْ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ. عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ، إِلاَّ بِحَقِّهِ، وَحِسَابُهُ عَلَى اللَّهِ ". فَقَالَ وَاللَّهِ لأُقَاتِلَنَّ مَنْ فَرَّقَ بَيْنَ الصَّلاَةِ وَالزَّكَاةِ، فَإِنَّ الزَّكَاةَ حَقُّ الْمَالِ، وَاللَّهِ لَوْ مَنَعُونِي عِقَالاً كَانُوا يُؤَدُّونَهُ إِلَى رَسُولِ اللَّهِ ﷺ لَقَاتَلْتُهُمْ عَلَى مَنْعِهِ. فَقَالَ عُمَرُ فَوَاللَّهِ مَا هُوَ إِلاَّ أَنْ رَأَيْتُ اللَّهَ قَدْ شَرَحَ صَدْرَ أَبِي بَكْرٍ لِلْقِتَالِ فَعَرَفْتُ أَنَّهُ الْحَقُّ. قَالَ ابْنُ بُكَيْرٍ وَعَبْدُ اللَّهِ عَنِ اللَّيْثِ عَنَاقًا. وَهْوَ أَصَحُّ.