Bukhari

Hadith Commentary:
(1)
What is meant by accusing someone of fisq (open sinfulness) and kufr (disbelief) is to call another person "O fasiq" (open sinner) or "O kafir" (disbeliever).
If the accused person is not actually a fasiq or kafir, then the accusation of fisq or kufr returns upon the one who uttered it, meaning he himself becomes a fasiq or kafir.
Attributing fisq and kufr to someone is tantamount to insulting them.
The gravity of this can be gauged from the fact that if fisq or kufr is not found in the accused, then the one who says it himself becomes a fasiq or kafir.
(2)
This absolutely does not mean that if the cause of kufr or fisq is found in someone, then there is no sin in calling him a fasiq or kafir. Rather, Hafiz Ibn Hajar rahimahullah states that there is some detail in this:
"If the intention is to shame him, or to give him a bad reputation, or to cause him harm, then doing so is haram (forbidden), because a person has been commanded to conceal faults.
As long as gentle treatment is possible, it is haram to be harsh.
Sometimes such an action becomes the cause of his misguidance and persistence upon it. But if sincerity and advice are intended by mentioning his condition to him or to someone else, then doing so is permissible."
(Fath al-Bari: 10/572)
And Allah knows best.