What is the ruling on one who sees an evil and does not change it?

Changing evil is obligatory by consensus. It is a collective obligation (fard kifaya) and may become an individual obligation (fard ayn) for some people. The change should be done by hand; if unable, then by tongue; and if unable, then by heart, which is the weakest of faith. It is a condition for the one forbidding evil to be knowledgeable about the evil. If it is among the apparent obligations or well-known prohibitions, then the general Muslim populace knows it. But if it involves intricate actions or sayings, then it is for the people of knowledge.