1
إِذَا عَطَسَ أَحَدُكُم فَلْيَقُلِ الْحَمْدُ لِلَّهِ, وَلْيَقُلْ لَهُ أَخُوهُ أَوْ صَاحِبُهُ: يَرْحَمُكَ اللَّهُ, فَإِذَا قَالَ لَهُ: يَرحَمُكَ اللَّهُ, فَلْيَقُلْ: يَهْدِيكُمُ اللَّهُ وَيُصْلِحُ بَالَكُمْ
(When one of you sneezes he should say:(All praise if for Allah.) and his brother or companion should say to him: (May Allah have mercy upon you.) and he (i.e. the one who sneezed) replies back to him:(May Allah guide you and rectify your condition.))
Source: Al-Bukhari 7/125.