The person who responds to someone's sneeze should say, Yarhamukallah. Then the sneezer will say again -

يَرْحَمُنَا اللَّهُ وَإِيَّاكُمْ، وَيَغْفِرُ لَنَا وَلَكُمْ

yar'hamunallahu wa iyyakum wa yaghfiru lanaa walakum

May Allah shower his mercy upon you and us and forgive you and us.

Reference: Adabul Mufrad: 941