Sahih Bukhari
Good Manners and Form (Al-Adab)
Volume: 008 / Book Number: 073 / Hadith Number: 240
Narrated 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."