Abu Hurayra reported that the Prophet, may Allah bless him and
grant him peace, said, "Allah loves the sneeze and dislikes yawning. When
one you sneezes and praises Allah, it is a duty for every Muslim who hears
him to say, 'May Allah have mercy on you.;' As for the yawn, it comes from
Shaytan. When one of you yawns, he should repress it as much as possible.
When one of you yawns, Shaytan laughs at him."