Ubaydullah bin Abdullah bin Utbah bin Mas'ud narrates from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that after the death of the Messenger of Allah (peace and blessings be upon him), when Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) became caliph and some of the Arabs disbelieved, Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said to Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him): How can you fight the people when the Messenger of Allah (peace and blessings be upon him) has said: "I have been commanded to fight against the people until they say 'La ilaha illallah' (There is no god but Allah). Whoever says 'La ilaha illallah' (There is no god but Allah) has protected his wealth and his life from me, except for the rights of Islam, and his reckoning is with Allah"? Sayyiduna Abu Bakr (may Allah be pleased with him) said: By Allah! I will fight whoever makes a distinction between prayer and zakah, for zakah is the right due on wealth. By Allah! If they withhold from me even a hobbling rope that they used to give to the Messenger of Allah (peace and blessings be upon him), I will fight them for withholding it. Sayyiduna Umar (may Allah be pleased with him) says: By Allah! I recognised that Allah had opened the chest of Abu Bakr (may Allah be pleased with him) to fighting, and I knew that it was the truth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18626