One should not base his actions on speculations. ...The implication of this directive is that it is not right for a Muslim to make bad estimations about other Muslims, or make allegations against others or take some action against others without proper knowledge of what the matter is or spread rumours merely on the basis of speculation.
The guidance which emerges from this directive is that a person should not become so mentally sick as to think ill of others; on the contrary, he should always think well of others. If the deed or words of a person induce him to think ill of him, he should try as far as he can to make a good justification, if it can be made. He should only think contrarily when he is not able to make any sound justification. It is better to think positively of a person who deserves to be thought of negatively than to think negatively of a person who deserves to be thought of positively.
Here the prohibition actually relates to being inquisitive for an evil purpose. In other words, here the intention of a person is to try to find some fault or mistake in the private life of another person and have access to the secrets of his family and family life. Sometimes, the motive of this probing is jealousy for it soothes a person to find out such facts about someone he considers to be his rival.
Moreover, the word used is لَمْز which includes other meanings as finding faults in others, ridiculing and mocking others, blaming others, making some one a target of objections whether openly or in secret. Obviously, in all these things the motive is considering oneself to be superior and others as inferior and mediocre.