Q: What is the ruling on a Muslim who describes his brother as being stupid or a fool? What is the ruling on describing a group of people as being weak in Iman (Faith/belief), and what if they really are?
A: It is prescribed for the Mu’min (believer) to address his Muslim brothers with good words and with their names. When he wants to advise them, he should do so in the nicest manner that is most likely to be accepted and to maintain Islamic brotherhood. Allah (Glorified be He) says: The believers, men and women, are Auliyâ’ (helpers, supporters, friends, protectors) of one another Auliyâ’ is the opposite of enemy. The Auliyâ’ address brothers in a way that they like and do not detest. The Prophet (peace be upon him) said: Righteousness is a good attitude. (Related by Muslim in his Sahih (authentic) Book of Hadith) He (peace be upon him) also said: You will not be agreeable to people by means of money. Rather, you will be agreeable to them by means of wide smile on your faces and good conduct. (Related by Abu Ya`ly and graded as Sahih by Al-Hakim)
He (peace be upon him) said: “The Mu’min is not one given to cursing, slandering, obscenity or abusiveness.”Related by Al-Tirmidhy and Al-Hakim classed it as Sahah. Indeed, his Sanad (chain of narrators) is reliable.