When the sneezer (Yarhamukallah) said, he replied -

يَهْدِيْكُمُ اللَّهُ وَيُصْلِحُ بَالَكُمْ

Yahdeekum-ullaahu wa yuslihu baalakum

May Allah guide you and set your affairs in order. Special instructions have been given in the hadith to answer the sneeze dua like answering the salutation. These Sunnahs are neglected in our society.

Reference: Sahih. Tirmidhi: 2741