Abu Hurayra said, "Two men sat in the presence of the Prophet mayAllah bless him and grant him peace, and one of them was from a noble familythan the other. The nobler of the two sneezed and did not praise Allah,so the Prophet did not ask for mercy for him. Then the other man sneezedand praised Allah, so the Prophet, may Allah bless him and grant him peace,asked for mercy on him. The noble man said, 'I sneezed in your presenceand you did not ask for mercy for me. This other than sneezed and you askedfor mercy on him.' The Prophet said, 'This man mentioned Allah, so I mentionedhim. You forgot Allah, so I forgot you.'"
English reference: Book 40, Hadith 932
Arabic reference: Book 1, Hadith 932