Short answer
Yes. Secretly reading another adult's messages, emails or phone is spying (tajassus), which the Quran forbids. This includes a spouse, friend or sibling. Exceptions are narrow: parents supervising young children, a real threat of serious harm, or an employer checking work devices under clear rules.
Spying on Muslims and searching for their private faults is haram. Looking into someone's private messages without permission falls under this. Parents may supervise their minor children for their protection.
The four schools agree on this.
Explanation
Allah says in Surah al-Hujurat: avoid much suspicion, for some suspicion is sin, and do not spy and do not backbite. The Prophet ﷺ warned those who chase the private faults of Muslims that Allah will chase their faults and expose them, even inside their homes. He also said that whoever listens to people's conversation when they dislike it will have molten lead poured into his ears on the Day of Judgement.
Common situations:
- Spouses: marriage does not give you the right to secretly search your spouse's phone. If you have serious doubts, talk openly, ask for honesty, or seek help from a counsellor or imam. Couples who agree to share access by choice are free to do so.
- Friends and siblings: reading their chats, or screenshotting and sharing them, is a breach of trust.
- Parents and children: parents are responsible for young children and may set controls and check their devices to protect them. With older teens, open rules and trust usually work better than secret spying.
- Real danger: if there is a clear sign of serious harm, such as abuse, self-harm or a planned crime, looking to prevent it is allowed, limited to what is needed.
- Work: an employer may monitor company devices under policies employees know about.
If you have already looked, do not spread what you saw. Repent, and if it affects your relationship, deal with it honestly. Private messages people share with you are a trust; see amanah. The four schools agree on this ruling.
The evidence
Quran, Surah Al-Hujuraat 49:12
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ ٱجْتَنِبُوا۟ كَثِيرًۭا مِّنَ ٱلظَّنِّ إِنَّ بَعْضَ ٱلظَّنِّ إِثْمٌۭ ۖ وَلَا تَجَسَّسُوا۟ وَلَا يَغْتَب بَّعْضُكُم بَعْضًا ۚ أَيُحِبُّ أَحَدُكُمْ أَن يَأْكُلَ لَحْمَ أَخِيهِ مَيْتًۭا فَكَرِهْتُمُوهُ ۚ وَٱتَّقُوا۟ ٱللَّهَ ۚ إِنَّ ٱللَّهَ تَوَّابٌۭ رَّحِيمٌۭ ﴿١٢﴾
O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful.
Spying on others is forbidden.
Sahih al-Bukhari 6064
Sahih... ولا تجسسوا ولا تحسسوا ...
"Do not spy, and do not pry."
Searching into others' private affairs is forbidden.
Sunan Abi Dawud 4880
Grading to be checked... لا تغتابوا المسلمين ولا تتبعوا عوراتهم ...
"Do not backbite Muslims and do not chase their private faults."
Seeking out others' hidden faults is sinful.
Sahih al-Bukhari 7042
Sahih... ومن استمع إلى حديث قوم وهم له كارهون ...
"Whoever listens to people's conversation when they dislike it..."
Prying into private talk carries a severe warning.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Hazr wa al-Ibaha. Spying on Muslims and exposing their faults is forbidden.
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.