Sahih Muslim Book 42, Hadith Number 7128.

Chapter : Sneezing and the disapproval of yawning.

Iyas b. Salama b. al-Akwa reported that his father reported to him that he heard Allah’s Apostle (may peace be upon him) as saying: A person sneezed in his presence and he said to him: May Allah have mercy upon you. And he then sneezed for the second time and Allah’s Messenger (may peace be upon him) said to him: He is suffering from cold (and no response is necessary).