It was narrated from Qais bin Tihfah Al-Ghifari that his father said:
“The Messenger of Allah(ﷺ) found me sleeping in the masjid on my stomach. He nudged me with his foot and said: ‘Why are you sleeping like this? This is a kind of sleep that Allah dislikes,’ or ‘that Allah hates.'"
Is it bad to sleep on your stomach? The short answer is “yes.”