Musnad Ahmad 30:1227

Rab'i ibn Ibrahim narrated to us, Abd al-Rahman narrated to us, from Sa'id ibn Abi Sa'id, from Abu Hurayrah, who said: Two men sneezed in the presence of the Prophet, may God bless him and grant him peace. One of them was more noble than the other. The noble one sneezed and did not praise God, so the Prophet, may God bless him and grant him peace, did not say "Yarhamuk Allah" (may God have mercy on you) to him. The other one sneezed and praised God, so the Prophet, may God bless him and grant him peace, said "Yarhamuk Allah" to him. The noble one said: "I sneezed in your presence and you did not say 'Yarhamuk Allah' to me, and this one sneezed in your presence and you said 'Yarhamuk Allah' to him." He said: "Indeed, this one remembered God, so I remembered him; and you forgot God, so I forgot you."