Musnad Abu Ya'la al-Mawsili 160:197

Two men sat with the Messenger of Allah (may Allah bless him and grant him peace), one of them being more noble than the other. One of them sneezed and praised Allah, so the Messenger of Allah (may Allah bless him and grant him peace) said "Yarhamuk-Allah" (may Allah have mercy on you) to him. Then the other sneezed, but he did not praise Allah, so the Prophet (may Allah bless him and grant him peace) did not say "Yarhamuk-Allah" to him. The noble one said, "This one sneezed and you said 'Yarhamuk-Allah' to him, and I sneezed and you did not say 'Yarhamuk-Allah' to me." He said: The Prophet (may Allah bless him and grant him peace) said, "Indeed, this one remembered Allah, so I remembered (him), and you forgot—meaning Allah—so I forgot you."