Praise be to Allah.
If a Muslim insults his fellow Muslim and puts him down, then he has committed a deed that is obviously prohibited, and has committed one of the sins of the tongue, which would lead to accumulation of bad deeds and would undermine his good deeds.
Allah, may He be exalted, says (interpretation of the meaning):
“O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them. And do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers”
It was narrated from ‘Abdullah ibn Mas‘ood (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Insulting a Muslim is an evil action and fighting him is disbelief (kufr).”
Narrated by al-Bukhaari (no. 48) and Muslim (64).
It was also narrated from him (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: The believer is not given to criticising people or given to cursing or foul-mouthed or shameless.” Narrated by at-Tirmidhi (1977). He said: It is hasan ghareeb. Classed as saheeh by al-Albaani in Saheeh at-Tirmidhi.
Al-Mubaarakfoori (may Allah have mercy on him) said:
“The believer” means the perfect believer
“given to criticising people” means given to pointing out their faults.
“given to cursing” – the form of the word in the original Arabic is an emphatic form, because the perfect believer can rarely avoid this shortcoming altogether.
“foul-mouthed” refers to one who says or does things that foul or shameless. In an-Nihaayah it says: That is, one who commits foul or shameless words or deeds.
And it was said that this word refers to the slanderer. What appears to be the case is that it refers to abhorrent slander that should not be mentioned.
“or shameless” – al-Qaari said: This refers to the one who feels no shame or embarrassment.
End quote from Tuhfat al-Ahwadhi (6/11).
This applies if by describing his brother as a Jew he meant that he is miserly or greedy by way of slander and reviling him to his face, or by way of backbiting if he was not present.
But if he meant to say that he is a follower of the religion of the Jews, then this is more emphatically prohibited.
The Prophet (blessings and peace of Allah be upon him) said: “If a man says to his brother, ‘O you disbeliever (kaafir)!’, that description will apply to one of them.” Narrated by al-Bukhaari (6104) and Muslim (60).
This is by way of showing how serious such talk is.
See the answer to question no. 212566.
If he says: O Jew, O Christian, O Magian (Zoroastrian), or O son of a Jew, there is no hadd punishment for that, because accusing someone of being a disbeliever does not come under the heading of slander, and it does not harm the one who is so accused if his being Muslim is known to everyone. But the one who says that should be given a disciplinary punishment (ta‘zeer), because calling a Muslim a disbeliever is haraam, and by committing a haraam action the one who does that deserves a disciplinary punishment.
End quote from al-Mabsoot (9/126).
Ibn Hazm (may Allah have mercy on him) said: Accusing someone of being a disbeliever is haraam, although there is no hadd punishment for it; rather it is subject to a disciplinary punishment (ta‘zeer) only, because of the offence caused; it is an evil act and changing evil is obligatory, because of the command of the Messenger of Allah (blessings and peace of Allah be upon him) to that effect. And Allah, may He be exalted, is the source of strength.
End quote from al-Muhalla (12/250).
We should point out to the one who does that that what he has done is wrong, and such talk should not be taken lightly, because it is a transgression against the sanctity of his fellow Muslim.
And Allah knows best.