Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: When the Prophet (peace and blessings be upon him) passed away and Abu Bakr (may Allah be pleased with him) was made caliph, some of the Arabs refused to pay zakah. Umar (may Allah be pleased with him) said: O Abu Bakr! How can you fight these people, when the Prophet (peace and blessings be upon him) said: "I have been commanded to fight 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 by its right, and his reckoning is with Allah Almighty"? Abu Bakr (may Allah be pleased with him) said: By Allah! I will certainly 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 young she-goat which they used to give in the time of the Prophet (peace and blessings be upon him), I will fight them over it. Umar (may Allah be pleased with him) said: By Allah! I saw 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 قسم الصدقات / 13115