An imperative directs a person to do something. Even دَعْ (“leave it”) is grammatically an imperative although its instruction involves stopping an action. A typical prohibition uses لَا with a jussive present verb, as in the illustrative phrase لَا تَكْذِبْ (“do not lie”).
In Hadith 143, لَا يَرِيبُكَ is **negation**: “it does not cause you doubt.” It is not directly commanding the addressee to stop an action.
The distinction is therefore: دَعْ is an imperative; لَا تَكْذِبْ illustrates prohibition; and لَا يَرِيبُكَ is negation.