Musnad Abu Ya'la al-Mawsili 1:66

Muhammad ibn Isma'il ibn Abi Samina al-Basri and Muhammad ibn al-Muthanna narrated to us, they said: 'Amr ibn 'Asim al-Kilabi narrated to us, 'Imran narrated to us, from Ma'mar, from al-Zuhri, from Anas, who said: 'Umar said to Abu Bakr regarding the apostasy: "Did the Messenger of Allah (peace be upon him) not say: 'I have been commanded to fight the people until they say "There is no god but Allah." If they say it, their blood and their wealth are protected from me, except by its right, and their reckoning is with Allah'?" Abu Bakr said: "The Messenger of Allah (peace be upon him) said..." And in the hadith of Ibn Abi Samina, he said: "Abu Bakr said: 'The Messenger of Allah (peace be upon him) only said: "I have been commanded to fight the people until they say 'There is no god but Allah,' establish the prayer, and pay the zakat." By Allah, if they were to withhold from me a hobbling-rope [for a camel] that they used to pay, I would fight them for it until my soul joins Allah.'" 'Umar said: "So when I saw that Abu Bakr had resolved upon that, I knew that it was the truth."