Sahih Muslim Book 42, Hadith Number 7127.
Chapter : Sneezing and the disapproval of yawning.
Abu Burda reported: I visited Abu Musa, as he was in the house of the daughter of Fadl b. ‘Abbas. I sneezed but he did not respond to it (by saying): Allah may have mercy upon you. Then she sneezed and he (Fadl b. ‘Abbas) said: May Allah have mercy upon you. I came back to my mother and informed her about it, and when he came to her she said: My son sneezed in your presence and you did not say: “Allah may have mercy upon you, and she sneezed and you said for her: “May Allah have mercy upon you.” Thereupon he said: Your son sneezed but he did not praise Allah and I did not beg mercy of Allah for him and she sneezed and she praised Allah and so I said: May Allah have mercy upon you, as I heard Allah’s Messenger (may peace be upon him) as saying: When any one of you sneezes he should praise Allah and the other should say: May Allah have mercy upon you, and if he does not praise Allah, no mercy should be begged for him.