Is one who commits a permissible act, believing it to be forbidden, sinful, or one who neglects a supererogatory act, believing it to be obligatory, sinful, despite their knowledge of the error of their belief?

Whoever does an act that he believes to be unlawful, and then performs it, is sinful, even if the act was originally permissible. This is like someone who consumes something permissible, believing it to be unlawful, or has intercourse with his wife, believing her to be a foreign woman. Similarly, it applies to someone who intentionally abandons an obligatory act, despite being capable of performing it, believing it not to be obligatory. The fundamental principle is that the ignorant should ask the scholars about what they intend to do or abandon, in compliance with the words of Allah Almighty: "So ask the people of the message if you do not know."