Invocation for sneezing
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 Yarhamukallaah to you then you should say:
Yahdeekumul-laahu wa yuslihu baalakum.
May Allah guide you and set your affairs in order.
Reference: Al-Bukhari 7/125.