Reference: http://mufti.af.org.sa/ar/content/منكر-وجوب-الزكاة-كافر
Question: What is the ruling regarding the one who denies the obligation of zakaah?
Response: Whoever denies the obligation of zakaah, then he is a disbeliever. That is because its obligation is established in the Book [of Allaah] and the Sunnah, and by consensus.
Allaah (Ta’aala) says:
And zakaah has been mentioned in connection with salaah, so one is not correct without the other; and there are plenty of aayaat and ahaadeeth [related to the obligation of zakaah].
As for he who is lenient in extracting it [from his wealth], then he is indeed a faasiq, and the ruler [in authority] should discipline him, and take it from him by force.
We implore Allaah for the guidance of everyone.