It is narrated from Abu Hurairah (may Allah be pleased with him) that Umar (may Allah be pleased with him) said to Abu Bakr (may Allah be pleased with him) regarding those who refused to pay zakat: "Did not the Messenger of Allah (ﷺ) say, 'I have been commanded to fight the people until they say: ﴿La ilaha illallah﴾. When they say it, their blood and wealth will be protected from me except for its due right (i.e., legal retribution, etc.), and their reckoning is with Allah'?" Abu Bakr (may Allah be pleased with him) replied: "Is not zakat its due right, which they have refused to give?"
Hadith Referenceمسند الإمام الشافعي / كتاب الزكاة / 678
Hadith Takhrijاخرجه البخارى الزكاة، باب وجوب الزكاة (1399)، (1400) ومسلم ، الايمان، باب الأمر بقتال الناس حتى يقولوا.... الخ (20)۔