Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Lying on one’s belly is, indeed, a controversial matter.
2- While some scholars consider it disliked (makruh), others deem it permissible.
Responding to your question, Sheikh Kifah Mustapha, the Imam and Director of The Prayer Center of Orland Park and representative for Dar El Fatwa of Lebanon in the US, states:
Muslim scholars differ on the ruling of sleeping on one’s belly. Some of them view that it is makruh (disliked) or not preferred to sleep on one’s belly while others consider it mubah (allowed).
Many scholars say that the narrated Hadiths on sleeping on the belly do not reach the level of Sahih (authentic) at all.
Imam Al-Bukhari (may Allah have mercy on him) mentioned it in his At-Tarikh Al-Kabir and declared that it is not authentic.
The same view was held also by Ibn Abi Hatim in his al-I`ilal and Ad-Daraqutni in his al-I`ilal.
Among the scholars who spoke in detail about the weakness of these hadiths is Imam Abu Ishaq Al-Harbi in his Ikram Ad-Dayf, pages 42-48.
Imam Al-Albani (may Allah have mercy on him) brought it to the level of “hasan sahih” due to the many narrations on the topic. (See his Sahih At-Targhib wat-Tarhib 3/188).
I am for the view that it is not makruh to sleep on the belly. At the same time, it is wise to watch out children sleeping on their stomach for many cases of suffocation were reported.
Finally, having said this, I should also point out that the Sunnah is to sleep on your right side as mentioned in the authentic Hadith: “When the Prophet (peace and blessings be upon) goes to bed, he would sleep on his right side.” (Al-Bukhari)
Allah Almighty knows best.