Al-Bukhaari (6224) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said:
“When one of you sneezes, let him say, ‘Al-hamdu-Lillaah (Praise be to Allaah),’ and let his brother or companion say to him. ‘Yarhamuk Allaah (May Allaah have mercy on you).’ If he says, ‘Yarhamuk-Allaah,’ then let (the sneezer) say, ‘Yahdeekum Allaah wa yuslihu baalakum (May Allaah guide you and rectify your condition).’”