Authentic Supplication Number 234.

It is reported that the Prophet  said: "Allah likes sneezing, and he hates yawning. So if one of you sneezes, and praises Allah, then it is a duty upon anyone that hears him (239) to say L Yarhamuka 'allahu (240).

As for Yawning, it is of the shaytan, so if one of you is going to yawn, let him hold it back as much as possible, because if one of you yawns, shaytan laughs at him."

Footnotes.
(223) Shaikh Al-Albani said in his footnottes to The Authentic of Good Sayings (Saheeh Al-Kalim At-Tayyib) that this is evidence that anyone that hears the sneezing person say "'Al-hamdu lil-lah" must reply. The belief that if one person says it the rest do not have to is a misconception. This is different than the greeting with As-Salam due to the preceeding hadeeth #221.

Reporters.
Reported by Al-Bukhari, and is #161 in The Authentic of Good Sayings.