Narrated Abu Huraira: The Prophet said, " If anyone of you sneezes, he should say 'Al-Hamdulillah' (Praise be to Allah), and his (Muslim) brother or companion should say to him, 'Yar-hamuka-l-lah' (May Allah bestow his Mercy on you). When the latter says 'Yar-hamuka-llah", the former should say, 'Yahdikumul-lah wa Yuslih balakum' (May Allah give you guidance and improve your condition)."

— Sahih Bukhari, Volume 8, Book 73: Good Manners and Form (Al-Adab), Hadith #243