Sahih Bukhari Volume 2, Book 24, Hadith 1400

حَدَّثَنَا أَبُو الْيَمَانِ الْحَكَمُ بْنُ نَافِعٍ، أَخْبَرَنَا شُعَيْبُ بْنُ أَبِي حَمْزَةَ، عَنِ الزُّهْرِيِّ، حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ بْنِ مَسْعُودٍ، أَنَّ أَبَا هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَكَانَ أَبُو بَكْرٍ ـ رضى الله عنه ـ وَكَفَرَ مَنْ كَفَرَ مِنَ الْعَرَبِ فَقَالَ عُمَرُ ـ رضى الله عنه كَيْفَ تُقَاتِلُ النَّاسَ، وَقَدْ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَقُولُوا لاَ إِلَهَ إِلاَّ اللَّهُ‏.‏ فَمَنْ قَالَهَا فَقَدْ عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ إِلاَّ بِحَقِّهِ، وَحِسَابُهُ عَلَى اللَّهِ ‏"‏‏

Narrated By Abu Huraira : When Allah's Apostle died and Abu Bakr became the caliph some Arabs renegade (reverted to disbelief) (Abu Bakr decided to declare war against them), 'Umar, said to Abu Bakr, "How can you fight with these people although Allah's Apostle said, 'I have been ordered (by Allah) to fight the people till they say: "None has the right to be worshipped but Allah, and whoever said it then he will save his life and property from me except on trespassing the law (rights and conditions for which he will be punished justly), and his accounts will be with Allah.'