When you sneeze, then say
Alhamdu lillaahالْحَمْـدُ للهِ
May Allah have mercy upon you.
Your companion should say
يَرْحَمُـكَ الله
Yarhamukallaah
All praises and thanks are to Allah.
When someone says Yarhamukallaah to you, then you should say
يَهْـديكُـمُ اللهُ وَيُصْـلِحُ بالَـكُم
Yahdeekumul-laahu wa yuslihu baalakum
May Allah guide you and set your affairs in order.
Al-Bukhari 7/125.