Imam al-Shafi'i (may Allah have mercy on him) said: "And these are people who held firm to Islam but refused (to pay) zakah."
And he cited as evidence for this the incident of Sayyiduna Abu Bakr and Sayyiduna Umar (may Allah be pleased with them both).
Sayyiduna Abu Hurayrah (may Allah be pleased with him) says: After the Prophet (peace and blessings be upon him) passed away and Sayyiduna Abu Bakr (may Allah be pleased with him) became caliph, some of the Arab tribes disbelieved. Sayyiduna Umar (may Allah be pleased with him) said to Sayyiduna Abu Bakr (may Allah be pleased with him): How will you fight them, when the Prophet (peace and blessings be upon him) has said: "I have been commanded to fight people until they say the testimony 'La ilaha illallah' (There is no god worthy of worship except Allah); when they affirm it, they have protected their wealth and their lives from me, except by its right, and their reckoning is with Allah"? Sayyiduna Abu Bakr (may Allah be pleased with him) replied: "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 used to give the Prophet (peace and blessings be upon him) even a hobbling rope ('iqal) in zakah and now refuse it, I will certainly fight them." Sayyiduna Umar (may Allah be pleased with him) says: By Allah, it was nothing other than that I saw that Allah had opened the chest of Sayyiduna Abu Bakr (may Allah be pleased with him) to fighting, and so I knew that it was the truth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16730