Reference: Fataawa Ramadhaan v2 p903 no.887
Fiqh al-Ibaadaat libni ‘Uthaymeen p213
Question: What is the ruling for denying Zakaatul-Fitr and how is the denier to be dealt with?
Response: It’s denial is prohibited! Because it is egressing from that which the Messenger of Allaah ﷺ has legislated, as has just preceded from the hadeeth of Ibn ‘Umar (radhi-yAllaahu ‘anhumaa) [who said]: “The Prophet ﷺ legislated Zakaatul-Fitr…”, and it is known that leaving that which is obligatory is prohibited, and in [doing so] is sin and disobedience.
