Girls, is it true that babies under one year old shouldn't sleep on their…

Girls, is it true that babies under one year old shouldn't sleep on their stomachs? My baby is seven months old, and lately he often turns over on his stomach while sleeping. When he falls asleep in his crib, he only sleeps on his tummy or on his side. When he falls asleep in my arms, I put him on his back, but sometimes I wake up at night and he's sleeping on his stomach. I turn him onto his back. Am I doing it right? Or should I leave it as it is?