Sahih Muslim: Hadith 4428

The command to fight the people until they say "La ilaha illallah Muhammad Rasul-Allah", and establish Salat, and pay the Zakat, and believe in everything that the prophet (ﷺ) brought. Whoever does that, his life and his wealth are protected except by its right, and his secrets are entrusted to Allah, the most high. Fighting those who withhold Zakat or other than that is one of the duties of Islam and the Iman should be concerned with the Laws of Islam