When the Messenger of Allah ﷺ died and Abu Bakr ؓ became (caliph) after him, and some of the Arabs apostatized,ʻUmar ؓ said: O Abu Bakr, how can you fight the people when the Messenger of Allah ﷺ said: ʿI have been ordered to fight the people until they bear witness that there is no god except Allah. Whoever bears witness that there is no god except Allah has protected his wealth and his life from me, except in cases dictated by Islamic law, and their reckoning will be with Allahʿ? Abu Bakr ؓ said: by Allah I shall certainly fight the one who separates prayer and zakah, for zakah is what is due from wealth. By Allah, whoever withholds from me a goat that they used to give (in zakah) to the Messenger of Allah ﷺ, I shall fight them for withholding it. 'Umar said: By Allah, as soon as I realized that Allah had opened the heart of Abu Bakr to fighting, I realized that it was the right thing to do. (Using translation from Aḥmad 117)   

أَخْبَرَهُ قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللهِ ﷺ وَاسْتُخْلِفَ أَبُو بَكْرٍ ؓ بَعْدَهُ وَكَفَرَ مَنْ كَفَرَ مِنَ الْعَرَبِ قَالَ عُمَرُ ؓ يَا أَبَا بَكْرٍ كَيْفَ تُقَاتِلُ النَّاسَ وَقَدْ قَالَ رَسُولُ اللهِ ﷺ أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَقُولُوا لَا إِلَهَ إِلَّا اللهُ فَمَنْ قَالَ لَا إِلَهَ إِلَّا اللهُ عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ إِلَّا بِحَقِّهِ وَحِسَابُهُ عَلَى اللهِ ؟ قَالَ أَبُو بَكْرٍ وَاللهِ لَأُقَاتِلَنَّ مَنْ فَرَّقَ بَيْنَ الصَّلَاةِ وَالزَّكَاةِ فَإِنَّ الزَّكَاةَ حَقُّ الْمَالِ وَاللهِ لَوْ مَنَعُونِي عَنَاقًا كَانُوا يُؤَدُّونَهَا إِلَى رَسُولِ اللهِ ﷺ لَقَاتَلْتُهُمْ عَلَى مَنْعِهَا قَالَ عُمَرُ فَوَاللهِ مَا هُوَ إِلَّا أَنْ رَأَيْتُ اللهَ قَدْ شَرَحَ صَدْرَ أَبِي بَكْرٍ لِلْقِتَالِ فَعَرَفْتُ أَنَّهُ الْحَقُّ