What is the ruling on saying "So-and-so is a Jamī"?

It is not permissible to categorize Muslims and Ahl al-Sunnah with designations for which Allah has sent down no authority, because that leads to division and hatred. Allah has commanded unity and concord and forbidden division and disagreement, as He Almighty said: "O you who have believed, fear Allah as He should be feared and do not die except as Muslims [in submission to Him]. And hold firmly to the rope of Allah all together and do not become divided."

Among the rights of a Muslim upon his brother Muslim is to show him loyalty and advise him with wisdom regarding any sin he observes. Labeling one another with such titles is not from good counsel; rather, it leads to disaffection between hearts. The youth of Islam should learn their religion, act upon it, and invite others to it, and be keen on uniting their word and abandoning discord.