Short answer
It is not haram, but it is disliked (makruh). The Prophet ﷺ saw a man lying on his stomach and said that this is a way of lying down that Allah does not love. The sunnah is to sleep on your right side. If you need to lie on your stomach for a medical reason, there is no blame.
Lying face down without need is makruh. The sunnah is to lie on the right side, facing the qibla where easy, after wudu and the sleeping duas.
The four schools agree on this.
Explanation
Abu Hurayra reported that the Prophet ﷺ saw a man lying on his stomach and said: 'This is a way of lying that Allah does not love.' On that basis the scholars of the four schools agree that it is disliked for men and women when there is no need.
The sunnah way to sleep was taught by the Prophet ﷺ to al-Bara' ibn Azib: make wudu as for prayer, then lie on your right side and say the dua he taught. You can find the duas in dua before sleeping and other bedtime habits in Sunnahs of daily life.
Common questions:
- I turn onto my stomach while asleep. There is no sin for what happens while you sleep. Start on your right side and leave the rest.
- Back pain or a medical condition. If a doctor advises it, or it is the only way you can rest, there is no blame.
- Lying on your stomach while awake, for reading or using your phone, is also covered by the dislike, though briefly doing so is a light matter.
- Babies should sleep however medical safe-sleep advice says. The dislike is for adults.
This is a matter of good manners and following the sunnah, not a major issue. Do not worry over it, just build the habit of starting on your right side.
The evidence
Jami' at-Tirmidhi 2768
Graded hasan sahih... إن هذه ضجعة لا يحبها الله ...
"This is a way of lying down that Allah does not love."
Lying on the stomach is disliked.
Sahih al-Bukhari 247
Sahih... ثم اضطجع على شقك الأيمن ...
"Then lie down on your right side."
The sunnah is to sleep on the right side.
Where the ruling is recorded
- Sunan al-Tirmidhi, Kitab al-Adab, chapter on the dislike of lying on the stomach. The chapter heading itself records the ruling of dislike.
This answer is pending review by the scholars on our team. 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.