Leave that which makes you doubt for that which does not make you doubt

Prophet (sallallaahu ‘alayhe wa sallam) said, what means, “Leave that which makes you doubt for that which does not make you doubt.”
And he said, what means, “The halaal is clear, and the haraam is also clear. Between the two are things are grey areas that many people do not know about. So anyone stays on guard against those grey areas will surely remain safe in his religion and his honor will stay intact. But whoever delves into the grey areas will fall into the haraam, like the case of a shepherd who continually lets his flock graze right near a border (he is not supposed to transgress)…”
Both of these hadeeths are in a small compilation called “Al-Arba’oon an-Nawawiyyah” (The 40 Hadeeth of an-Nawawi), which would be a great book to pick up to learn some basic and important hadeeths.