Sunan an-Nasa'i, Book 37, Hadith 4 (In-Book) ⓘ

It was narrated that Anas bin Malik said: "When the Messenger of Allah [SAW] died, the 'Arabs apostatized, so 'Umar said: 'O Abu Bakr, how can you fight the 'Arabs?' Abu Bakr said: 'The Messenger of Allah [SAW] said: "I have been commanded to fight the people until they bear witness to La ilaha illallah (there is none worthy of worship except Allah) and that I am the Messenger of Allah, and they establish Salah and pay Zakah." By Allah, if they withhold from me a young goat that they used to give to the Messenger of Allah [SAW], I will fight them for it.' 'Umar said: 'By Allah, as soon as I realized how certain Abu Bakr was, I knew that it was the truth.'"

أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا عَمْرُو بْنُ عَاصِمٍ، قَالَ حَدَّثَنَا عِمْرَانُ أَبُو الْعَوَّامِ، قَالَ حَدَّثَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ لَمَّا تُوُفِّيَ رَسُولُ اللَّهِ صلى الله عليه وسلم ارْتَدَّتِ الْعَرَبُ فَقَالَ عُمَرُ يَا أَبَا بَكْرٍ كَيْفَ تُقَاتِلُ الْعَرَبَ فَقَالَ أَبُو بَكْرٍ إِنَّمَا قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَشْهَدُوا أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنِّي رَسُولُ اللَّهِ وَيُقِيمُوا الصَّلاَةَ وَيُؤْتُوا الزَّكَاةَ ‏" ‏ ‏.‏ وَاللَّهِ لَوْ مَنَعُونِي عَنَاقًا مِمَّا كَانُوا يُعْطُونَ رَسُولَ اللَّهِ صلى الله عليه وسلم لَقَاتَلْتُهُمْ عَلَيْهِ ‏.‏ قَالَ عُمَرُ فَلَمَّا رَأَيْتُ رَأْىَ أَبِي بَكْرٍ قَدْ شُرِحَ عَلِمْتُ أَنَّهُ الْحَقُّ ‏.‏