كتاب تحريم الدم
Read the English translation first, with the original Arabic directly underneath.
It was narrated that Abu Hurairah said:
"When the Messenger of Allah [ﷺ] died, and Abu Bakr (became Khalifah) after him, and the 'Arabs reverted to Kufr, 'Umar said: 'O Abu Bakr, how can you fight the people when the Messenger of Allah [ﷺ] said: I have been commanded to fight the people until they say La ilaha illallah, and whoever says La ilaha illallah, his wealth and his life are safe from me, except for a right that is due, and his reckoning will be with Allah, the Mighty and Sublime?' Abu Bakr said: 'I will fight whoever separates Salah and Zakah, for Zakah is the compulsory right to be taken from wealth. By Allah, if they withhold from me a young goat that they used to give to the Messenger of Allah [ﷺ], I will fight them for withholding it.' 'Umar said: 'By Allah, as soon as I saw that Allah has expanded the chest of Abu Bakr to fighting, I knew that it was the truth.'"
أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَقُولُوا لاَ إِلَهَ إِلاَّ اللَّهُ فَمَنْ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ فَقَدْ عَصَمَ مِنِّي مَالَهُ وَنَفْسَهُ إِلاَّ بِحَقِّهِ وَحِسَابُهُ عَلَى اللَّهِ عَزَّ وَجَلَّ