Short answer
Yes. Calling a Muslim 'kafir' (disbeliever) or 'munafiq' (hypocrite) as an insult is haram, so never do it. The Prophet ﷺ warned that if the person is not what you called him, the word comes back on you. Only a qualified scholar can judge that a specific person has left Islam.
Insulting a Muslim with 'kafir', 'munafiq' or similar words is haram and deserves discretionary punishment (ta'zir) in an Islamic court. In the relied-upon fatwa the speaker does not become a disbeliever through the insult alone, but if he calls a Muslim kafir believing that the Muslim's faith itself is disbelief, he commits kufr.
The four schools agree on this.
Explanation
The Prophet ﷺ said that when a man says to his brother 'O kafir', one of the two deserves it: if the other is as he said, and if not, it returns to the speaker. Scholars explain that 'it returns to him' means the sin and weight of the accusation fall back on him. It makes him a disbeliever only if he calls Islam itself disbelief. He also said that insulting a Muslim is fisq (open sin). The Quran forbids calling each other by offensive names.
Usama ibn Zayd once killed a man in battle after the man said La ilaha illallah, thinking he only said it to save himself. The Prophet ﷺ was deeply upset and asked, 'Did you cut open his heart to see?' Faith and hypocrisy are matters of the heart. You can judge what people do, but not what is inside them.
Where this comes up today:
- Online arguments: throwing 'kafir', 'munafiq' or 'murtad' at people who disagree with you.
- Differences of madhhab or practice: calling Muslims kafir over issues on which scholars differ.
- Family fights: calling a relative a hypocrite because they pray at home or do not wear hijab.
Saying that an act is kufr or that a belief is hypocrisy, in a teaching setting, is not the same as labelling a person. Scholars explain what is kufr so that people avoid it. Declaring a named person outside Islam (takfir) has strict conditions and is the job of a qualified mufti or court.
If you said it in anger, repent, take it back and apologise. The four schools agree on this ruling. See also what hypocrisy is.
The evidence
Quran, Surah Al-Hujuraat 49:11
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا يَسْخَرْ قَوْمٌۭ مِّن قَوْمٍ عَسَىٰٓ أَن يَكُونُوا۟ خَيْرًۭا مِّنْهُمْ وَلَا نِسَآءٌۭ مِّن نِّسَآءٍ عَسَىٰٓ أَن يَكُنَّ خَيْرًۭا مِّنْهُنَّ ۖ وَلَا تَلْمِزُوٓا۟ أَنفُسَكُمْ وَلَا تَنَابَزُوا۟ بِٱلْأَلْقَٰبِ ۖ بِئْسَ ٱلِٱسْمُ ٱلْفُسُوقُ بَعْدَ ٱلْإِيمَٰنِ ۚ وَمَن لَّمْ يَتُبْ فَأُو۟لَٰٓئِكَ هُمُ ٱلظَّٰلِمُونَ ﴿١١﴾
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.
Offensive names and insults are forbidden.
Sahih al-Bukhari 6103
Sahih... إذا قال الرجل لأخيه يا كافر فقد باء به أحدهما ...
"If a man says to his brother 'O disbeliever', one of the two deserves it."
The label falls back on the speaker when it is false.
Sahih Muslim 60
Sahih... فقد باء بها أحدهما إن كان كما قال وإلا رجعت عليه ...
"Then one of the two deserves it: if it is as he said; otherwise it returns to him."
The accusation falls back on the speaker when the other person is not a disbeliever.
Sahih al-Bukhari 48
Sahih... سباب المسلم فسوق وقتاله كفر ...
"Insulting a Muslim is open sin, and fighting him is disbelief."
Insulting Muslims is a serious sin.
Sahih Muslim 96
Sahih... أفلا شققت عن قلبه ...
"Did you cut open his heart?"
No one can judge what is in another person's heart.
Where the ruling is recorded
- al-Durr al-Mukhtar, Kitab al-Hudud, chapter on ta'zir. Calling a Muslim 'kafir' and similar insults deserves ta'zir.
- Radd al-Muhtar (Ibn Abidin), Bab al-Murtadd. The speaker is not judged a disbeliever for an insult, unless he believes the Muslim's faith is kufr.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.