It was narrated from 'Ubaidullah bin 'Abdullah bin 'Utbah bin Mas'ud that Abu Hurairah said:

"When the Messenger of Allah (ﷺ) died and Abu Bakr (was appointed Khalifah) after him, and some of the 'Arabs disbelieved, 'Umar, may Allah be pleased with him, 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 of 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 him, said: "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 Mighty and Sublime, had opened the chest of Abu Bakr to fighting, then I knew that it was the truth.'" The wording is that of Ahmad.

أَخْبَرَنَا أَحْمَدُ بْنُ مُحَمَّدِ بْنِ مُغِيرَةَ، قَالَ حَدَّثَنَا عُثْمَانُ بْنُ سَعِيدٍ، عَنْ شُعَيْبٍ، عَنِ الزُّهْرِيِّ، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ، ح وَأَنْبَأَنَا كَثِيرُ بْنُ عُبَيْدٍ، قَالَ حَدَّثَنَا بَقِيَّةُ، عَنْ شُعَيْبٍ، قَالَ حَدَّثَنِي الزُّهْرِيُّ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُتْبَةَ بْنِ مَسْعُودٍ، أَنَّ أَبَا هُرَيْرَةَ، قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَكَانَ أَبُو بَكْرٍ بَعْدَهُ وَكَفَرَ مَنْ كَفَرَ مِنَ الْعَرَبِ قَالَ عُمَرُ رضى الله عنه يَا أَبَا 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 ‏

‏ ‏.‏ قَالَ أَبُو بَكْرٍ رضى الله عنه لأُقَاتِلَنَّ مَنْ فَرَّقَ بَيْنَ الصَّلاَةِ وَالزَّكَاةِ فَإِنَّ الزَّكَاةَ حَقُّ الْمَالِ وَاللَّهِ لَوْ مَنَعُونِي عَنَاقًا كَانُوا يُؤَدُّونَهَا إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم لَقَاتَلْتُهُمْ عَلَى مَنْعِهَا ‏.‏ قَالَ عُمَرُ فَوَاللَّهِ مَا هُوَ إِلاَّ أَنْ رَأَيْتُ أَنَّ اللَّهَ عَزَّ وَجَلَّ شَرَحَ صَدْرَ أَبِي بَكْرٍ لِلْقِتَالِ فَعَرَفْتُ أَنَّهُ الْحَقُّ ‏.‏ وَاللَّفْظُ لأَحْمَدَ ‏.‏