Musnad Ahmad 32:22

Mu'tamir ibn Sulayman narrated to us, he said: My father said: Anas ibn Malik narrated to us—I believe he said—: Two men sneezed in the presence of the Prophet, may Allah bless him and grant him peace. He said "Yarhamukallah" (may Allah have mercy on you) to one of them—or he said: "Sammitahu" (he invoked mercy for him)—and he left the other. It was said: Two men sneezed, and you invoked mercy for one and did not invoke mercy for the other. He said: "Indeed, this one praised Allah, the Mighty and Majestic."