The Messenger of Allah said: "I have been commanded to fight the people until they say La ilaha illallah (there is none worthy of worship but Allah). 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."
Sunan an-Nasa'i Book 37, Hadith 7
← Previous Hadith "The Messenger of Allah say such and such?' He said: 'By Allah, I do not separate Salah and Zakah, and I will fight whoever separates them.' So we fought alongside him, and we realized that that was the right thing."

Sunan an-Nasa'i Book 37, Hadith 6 Next Hadith → "When the Messenger of Allah , 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.'"

Sunan an-Nasa'i Book 37, Hadith 8