What is the ruling on one who commits a sin while ignorant of its prohibition, and what is the ruling on one who follows an erroneous fatwa; is he sinful?

Whoever commits a forbidden act out of ignorance of its prohibition is not sinful unless he was negligent in seeking the obligatory knowledge, in which case he is sinful due to his negligence. As for someone who sought a fatwa from someone he believed to be knowledgeable, and that person issued an erroneous fatwa, the sin is on the mufti, unless the error becomes clear to the questioner, in which case the sin attaches to him.