Abu Hurayrah reported that the Prophet (Peace and blessing of Allah be upon him) said: "When a person sneezes he should say: 'Praise be to Allah.' His brother or companion should reply: 'May Allah be merciful to you.' The first one should then say: 'May Allah guide you and order your affairs.' "

Imam al-Bukhari in Moral Teachings of Islam by Abdul Ali Hamid, translator