Short answer
Whistling is not haram, but whistling for fun or as a habit is makruh (disliked) in the Hanafi school, because the Quran describes whistling and clapping as the 'worship' of the idolaters at the Kaaba. Whistling for a need, such as calling an animal, a referee's whistle or a signal, is fine. The idea that whistling at night calls jinn or snakes has no basis.
Whistling with the mouth as a pastime or for music is makruh, as it resembles the practice of the idolaters and is idle play. Whistling for a real purpose, and using a whistle as a tool, is mubah (permitted).
The four schools agree on this.
Explanation
Allah describes the idolaters of Makkah: their prayer at the House was nothing but whistling and handclapping. The Companions and early scholars explained that they would whistle and clap around the Kaaba as a form of worship. Because of this verse, scholars dislike whistling as a pastime: it copies something the Quran mentions in blame, and it is idle play that adds nothing.
It is not haram, and no sin is written for whistling once without thinking. The Hanafi scholars put it in the category of makruh: better left, especially as a habit, in the mosque, or as a way of making music. Whistling a tune to replace a song does not make the song lawful.
Whistling for a need is a different matter:
- calling a dog, sheep or other animal, or getting someone's attention from a distance;
- a referee, coach, lifeguard or teacher using a whistle;
- safety whistles, sports signals and similar tools;
- whistling at someone to harass them is forbidden for that reason, not for the sound.
Many people have heard that whistling at night invites jinn, snakes or bad luck. There is no verse or authentic hadith that says this. It is a folk belief, and treating a sound as an omen of harm falls under the superstition the Prophet ﷺ rejected (see are superstitions haram). If you want protection at night, read the adhkar before sleeping, such as Ayat al-Kursi.
Scholars of the other schools say much the same: whistling as play is disliked, and for a need it is allowed.
The evidence
Quran, Surah Al-Anfaal 8:35
وَمَا كَانَ صَلَاتُهُمْ عِندَ ٱلْبَيْتِ إِلَّا مُكَآءًۭ وَتَصْدِيَةًۭ ۚ فَذُوقُوا۟ ٱلْعَذَابَ بِمَا كُنتُمْ تَكْفُرُونَ ﴿٣٥﴾
And their prayer at the House was not except whistling and handclapping. So taste the punishment for what you disbelieved.
The Quran blames the idolaters' whistling and clapping at the Kaaba, which is why scholars dislike whistling as a pastime.
Sahih al-Bukhari 5776
Sahih... لا عدوى ولا طيرة ويعجبني الفأل ...
"There is no contagion and no bad omen, and I like optimism (fa'l)."
Beliefs that whistling at night brings jinn or bad luck are omens without basis.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Hazr wa al-Ibaha. Discusses acts of idle play such as clapping and whistling and when they are disliked.
- Ahkam al-Quran (al-Jassas), Commentary on Surah al-Anfal, verse 35. Explains the whistling and clapping of the idolaters at the House.
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.