It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that when the Prophet (peace and blessings of Allah be upon him) passed away and after him Sayyiduna Abu Bakr (may Allah be pleased with him) was chosen as the Caliph, and those among the Arabs who could become disbelievers, became so, Sayyiduna Umar (may Allah be pleased with him) said to Sayyiduna Abu Bakr (may Allah be pleased with him): "How can you fight against these people when the Prophet (peace and blessings of Allah be upon him) said: 'I have been commanded to fight the people until they say «لا اله الا الله» , whoever says «لا اله الا الله» has protected his life and wealth from me, except for the right of Islam, and his reckoning is with Allah?'" Upon hearing this, Sayyiduna Abu Bakr (may Allah be pleased with him) said: "By Allah! I will certainly fight against the one who differentiates between prayer and zakat, because zakat is the due right of wealth. By Allah! If they withhold even a young goat which they used to give to the Messenger of Allah (peace and blessings of Allah be upon him), I will fight them for it." Sayyiduna Umar (may Allah be pleased with him) says: "I understood that Allah Almighty had opened the heart of Sayyiduna Abu Bakr (may Allah be pleased with him) regarding this matter, and I realized that his opinion was indeed correct."