source :Osoul Global Center
If a Muslim denies zakat and he is fully aware of Islamic rules and has access to scholars he is considered as an apostate. Only those who are new to Islam or have valid reason are not deemed to be unbelievers.
Anyone who denies the duty of zakat, such a person is considered an apostate, and the rulings of apostasy apply to him.
- Al-Nadawi, Al-Fiqh al-Muyassar, p. 122.