Sahih Bukhari Volume 9, Book 88, Hadith 6926

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ ابْنِ شِهَابٍ، أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، أَنَّ أَبَا هُرَيْرَةَ، قَالَ لَمَّا تُوُفِّيَ النَّبِيُّ صلى الله عليه وسلم وَاسْتُخْلِفَ أَبُو بَكْرٍ، وَكَفَرَ مَنْ كَفَرَ مِنَ الْعَرَبِ، قَالَ عُمَرُ 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.‏ عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ، إِلاَّ بِحَقِّهِ، وَحِسَابُهُ عَلَى اللَّهِ ‏"‏‏

Narrated By Abu Huraira : When the Prophet died and Abu Bakr became his successor and some of the Arabs reverted to disbelief, 'Umar said, "O Abu Bakr! How can you fight these people although Allah's Apostle said, 'I have been ordered to fight the people till they say: 'None has the right to be worshipped but Allah, 'and whoever said, 'None has the right to be worshipped but Allah', Allah will save his property and his life from me, unless (he does something for which he receives legal punishment) justly, and his account will be with Allah?' "