Musnad Ahmad 32:227

Yahya narrated to us, from al-Taymi, from Anas, who said: Two men sneezed in the presence of the Prophet, may Allah bless him and grant him peace, so he said "Yarhamukallah" (may Allah have mercy on you) to one of them and not the other. It was said to him: "Two men sneezed, yet you said 'Yarhamukallah' to one and not the other?" He replied: "This one praised Allah (said Alhamdulillah), while that one did not praise Allah." Yahya said: And perhaps he said: "This, or something similar."