Authentic Supplication Number 236.

The Messenger of Allah  said: "If one of you sneezes and praises Allah then shammitooh (244),but if he does not praise Allah then do not make tashmeet upon him".

Footnotes.
(244) Imperitive of tashmeet which means say:yarhamuka 'allah.

Reporters.
Reported by Muslim, and is #163 in The Authentic of Good Sayings.