Do Not Sleep On Your Stomach:


Takhfah ibn Qays al-Ghifaari (RadhiALLAHu Anho) said: When I was lying on my stomach in the Masjid just before dawn, a man nudged me with His foot and said 'This manner of lying is hated by ALLAH.'I looked and saw that it was the Messenger of ALLAH (salla ALLAHu alayhi wa sallam).
...
--Narrated by Ahmad
--Abu Dawood in al-Sunan, Kitaab al-Adab, no. 5040]


Ibn Takhfah al-Ghifaari that Abu Dharr (Radhi ALLAHu Anho) said: The Prophet (salla ALLAHu alayhi wa sallam) passed by me and I was lying on my stomach. He prodded me with his foot and said 'O Junaydib! This is how the people of Hell lie.'

--Ibn Majaah - 3724, al-Tirmidhi, no. 2798,
--Saheeh al-Jaami, 2270-2271