When must a layperson ask scholars, and is it obligatory to ask if one has previously committed an act that has no lingering effect and will not be repeated, or if one intends to say something whose ruling is unknown?

A Muslim must not undertake an action before knowing Allah's ruling on it. It is forbidden for him to undertake anything he suspects might be unlawful. What he does not intend to do, he is not obligated to ask about. As for what he doubts is permissible, he must inquire about it. If he has done something in the past and doubts its permissibility, he must ask about it so he can repent from it if it was unlawful and know its expiation. Whispers [waswas] should be dismissed and ignored, as excessive contemplation of them leads to great evil.