It is narrated from Hazrat Anas bin Malik (may Allah be pleased with him) that once in the gathering of the Prophet (peace and blessings be upon him), two men sneezed. The Prophet (peace and blessings be upon him) replied to one of them (by saying "Yarhamukallah") and left the other. Someone asked, "Two men sneezed, you replied to one of them, why did you not reply to the other?" He said, "He had said 'Alhamdulillah'."