Response: It’s denial is not permissible (muharram)! Because it is egressing from that which the Messenger of Allaah (sal-Allaahu `alayhe wa sallam) has legislated, as has just preceded from the hadeeth of Ibn ‘Umar (radhi-yallaahu ‘anhumaa) (who said): ((The Prophet (sal-Allaahu `alayhe wa sallam) legislated Zakaatul-Fitr…), and it is known that leaving off that which is obligatory is not permissible (haraam), and in (doing so) is sin and disobedience.
Shaykh Ibn ‘Uthaymeen
Fataawa Ramadhaan – Volume 2, Page 903, Fatwa No.887;
Fiqh al-Ibaadaat libni ‘Uthaymeen – Page 213.fatwaonline.com