Sahih Bukhari Volume 008, Book 073, Hadith Number 240.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 73. Good Manners and Form (Al-Adab)
Narated By Anas bin Malik : Two men sneezed before the Prophet. The Prophet said to one of them, "May Allah bestow His Mercy on you," but he did not say that to the other. On being asked (why), the Prophet said, "That one praised Allah (at the time of sneezing), while the other did not praise Allah."