Sahih Muslim Book 42, Hadith Number 7125.

Chapter : Sneezing and the disapproval of yawning.

Anas b. Malik reported that two persons sneezed in the presence of Allah’s Messenger (may peace be upon him). He (the Messenger of Allah) invoked mercy for one, and did not invoke for the other. The one for whom he had not prayed said: So and so sneezed and you said: May Allah have mercy upon you. I also sneezed but you did not utter these words for me. Thereupon he (the Holy Prophet) said: That person praised Allah, and you did not praise Allah.