Sunan an-Nasa'i 2443
It was narrated that Abu Hurairah said:
"When the Messenger of Allah died, and Abu Bakr became the Khalifah after him, and some of the 'Arabs reverted to disbelief. 'umar said to Abu Bakr: 'How can you fight the people when the Messenger of allah 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 illah, his wealth and his life safe from me, unless he deserves a legal punishment justly, and his reckoning will be with Allah?"'  Abu Bakr, may Allah be pleased with him, said: 'I will fight anyone who separates prayer and Zakah; 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, I will fight them for wiholding it.' 'Umar, may Allah be pleased with him, said: 'By Allah, it was as if I saw that Allah has opened the heart of Abu Bakr for fighting, and I knew that I was the truth."'
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، 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 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 ‏"‏ ‏.‏ فَقَالَ أَبُو بَكْرٍ رضى الله عنه 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 رضى الله عنه فَوَاللَّهِ مَا هُوَ إِلاَّ أَنْ رَأَيْتُ اللَّهَ شَرَحَ صَدْرَ أَبِي بَكْرٍ لِلْقِتَالِ فَعَرَفْتُ أَنَّهُ الْحَقُّ ‏.‏