"‏ 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 عَزَّ وَجَلَّ ‏"

Abu Hurairah said When the Messenger of Allah(ﷺ) died and Abu Bakr was made his successor after him and certain Arab clans apostatized. Umar bin Al Khattab said to Abu Bakr How can you fight with the people until they say “There is no God but Allah” so whoever says “There is no God but Allah”, he has protected his property and his person from me except for what is due from him, and his reckoning is left to allah. Abu Bak replied I swear by Allah that I will certainly fight with those who make a distinction between prayer and zakat, for zakat is what is due from property. I swear by Allah that if they were to refuse me a rope of camel (or a female kid, according to another version)which they used to pay the Messenger of Allah, I will fight with them over the refusal of it. Umar bin Al Khattab said I swear by Allah, I clearly saw Allah had made Abu Bakr feel justified in tighting and I recognized that it was right. Abu Dawud said This tradition has been transmitted by Rabah bin Zaid from Ma’mar and Al Zaubaidi from Al Zuhri has “If they were to refuse me a female kid.” The version transmitted by ‘Anbasah from Yunus on the authority of Al Zuhri has “a female kid”.