Musnad Ahmad 1:66

Muhammad ibn Yazid narrated to us, he said: Sufyan ibn Husayn informed us, from al-Zuhri, from Ubayd Allah ibn Abd Allah ibn Utbah ibn Mas’ud, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "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." He said: When the apostasy occurred, Umar said to Abu Bakr: "Will you fight them, when you have heard the Messenger of Allah, peace and blessings of Allah be upon him, say such and such?" Abu Bakr, may Allah be pleased with him, said: "By Allah, I will not separate between prayer and zakat, and I will surely fight those who separate between them." He said: So we fought alongside him, and we saw that as being the right course.