Sunan an-Nasa'i 3970
It was narrated that Abu Hurairah said:
"When the Messenger of Allah [SAW] died and Abu Bakr became the Khalifah after him, and some of the 'Arabs reverted to Kufr, 'Umar said to Abu Bakr: 'How can you fight the people when the Messenger of Allah [SAW] said: I have been commanded to fight the people until they say La ilaha illallah (there is none worthy of worship but Allah). Whoever says La ilaha illallah, his wealth and his life are safe from me except for a right that is due, and his reckoning will be with Allah.?' Abu Bakr said: 'By Allah, I will fight whoever separates Salah and Zakah, for Zakah is the compulsory right to be taken from wealth. By Allah, if they withhold from me a rope that they used to give to the Messenger of Allah [SAW], I will fight them for withholding it.' 'Umar, may Allah be pleased with him, said: 'By Allah, as soon as I realized that Allah has expanded the chest of Abu Bakr for fighting, I knew that it was the truth.'"
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ الزُّهْرِيِّ، أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَاسْتُخْلِفَ أَبُو بَكْرٍ وَكَفَرَ مَنْ كَفَرَ مِنَ الْعَرَبِ قَالَ عُمَرُ 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 عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ إِلاَّ بِحَقِّهِ وَحِسَابُهُ عَلَى اللَّهِ ‏"‏ ‏.‏ قَالَ 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 قَالَ عُمَرُ فَوَاللَّهِ مَا هُوَ إِلاَّ أَنِّي رَأَيْتُ اللَّهَ شَرَحَ صَدْرَ أَبِي بَكْرٍ لِلْقِتَالِ فَعَرَفْتُ أَنَّهُ الْحَقُّ ‏.‏