«إِذَا عَطَسَ أَحَدُكُم فَلْيَقُلِ الْحَمْدُ لِلَّهِ، وَلْيَقُلْ لَهُ أَخُوهُ أَوْ صَاحِبُهُ: يَرْحَمُكَ اللَّهُ، فَإِذَا قَالَ لَهُ: يَرحَمُكَ اللَّهُ، فَلْيَقُلْ: يَهْدِيكُمُ اللَّهُ وَيُصْلِحُ بَالَكُمْ».
When you sneeze, then say: All praises and thanks are to Allah. Your companion should say: May Allah have mercy upon you. When someone says Yarḥamukallāh to you then you should say: May Allah guide you and set your affairs in order.
SourceAl-Bukhari 7/125.