It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: I have been commanded to fight the people until they declare that there is no god but Allah. So when they recite this kalimah and declare it, they will protect their lives and wealth from me, and after that, their reckoning will be with Allah Almighty. When Sayyiduna Abu Bakr (may Allah be pleased with him) became the Caliph, some people became so apostate (that they refused to pay zakat). Sayyiduna Abu Bakr (may Allah be pleased with him) intended to fight them, but Sayyiduna Umar (may Allah be pleased with him) said: Why will you fight these people when they perform prayer? Sayyiduna Abu Bakr (may Allah be pleased with him) said: By Allah! I will certainly, certainly fight those who refuse to pay zakat. By Allah! If Allah and His Messenger have made even a young sheep or goat obligatory upon them as zakat, and if they do not give it, I will fight them for it. Sayyiduna Umar (may Allah be pleased with him) said: When I saw that Allah had opened the chest of Abu Bakr (may Allah be pleased with him) for fighting these people, I knew that this was the truth.