Sahih Muslim Book - 1, Hadith - 124

و حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا لَيْثُ بْنُ سَعْدٍ عَنْ عُقَيْلٍ عَنْ الزُّهْرِيِّ قَالَ أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ بْنِ مَسْعُودٍ عَنْ أَبِي هُرَيْرَةَ قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَاسْتُخْلِفَ أَبُو بَكْرٍ بَعْدَهُ وَكَفَرَ مَنْ كَفَرَ مِنْ الْعَرَبِ قَالَ عُمَرُ بْنُ الْخَطَّابِ لِأَبِي بَكْرٍ كَيْفَ تُقَاتِلُ النَّاسَ وَقَدْ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَقُولُوا لَا إِلَهَ إِلَّا اللَّهُ فَمَنْ قَالَ لَا إِلَهَ إِلَّا اللَّهُ فَقَدْ عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ إِلَّا بِحَقِّهِ وَحِسَابُهُ عَلَى اللَّهِ فَقَالَ أَبُو بَكْرٍ وَاللَّهِ لَأُقَاتِلَنَّ مَنْ فَرَّقَ بَيْنَ الصَّلَاةِ وَالزَّكَاةِ فَإِنَّ الزَّكَاةَ حَقُّ الْمَالِ وَاللَّهِ لَوْ مَنَعُونِي عِقَالًا كَانُوا يُؤَدُّونَهُ إِلَى رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَقَاتَلْتُهُمْ عَلَى مَنْعِهِ فَقَالَ عُمَرُ بْنُ الْخَطَّابِ فَوَاللَّهِ مَا هُوَ إِلَّا أَنْ رَأَيْتُ اللَّهَ عَزَّ وَجَلَّ قَدْ شَرَحَ صَدْرَ أَبِي بَكْرٍ لِلْقِتَالِ فَعَرَفْتُ أَنَّهُ الْحَقُّ.

It was narrated that Abu Hurairah said: "When the Messenger of Allah (s.a.w) died and Abu Bakr succeeded (as Khalifah) after him, and some of the Arabs reverted to Kufr, 'Umar bin Al-Khattab said to Abu Bakr: 'How can you fight the people when the Messenger of Allah (s.a.w) said: 'I have been commanded to fight the people until they say La ilaha illallah (none has the right to be worshiped but Allah), and whoever says La ilaha illallah, his wealth and his life are protected from me except for a right that is due, and his reckoning will be with Allah"?' Abu Bakr said: 'By Allah! I will most certainly fight those who separate Salat and Zakat, for Zakat is what is due on wealth. By Allah, if they withhold from me a rope that they used to give to the Messenger of Allah (s.a.w), I will fight them for withholding it.' 'Umar bin Al-Khattab said: 'By Allah, as soon as I saw that Allah had opened Abu Bakr's heart to the idea of fighting, I knew that he was right."'