"When one of you sneezes he should say:
All praises and thanks are to Allah.
...and his brother or companion should say to him:
May Allah have mercy upon you.
...and he [i.e. the one who sneezed] replies back to him:
يَهْدِيكُمُ اللَّهُ وَيُصْلِحُ بَالَكُم
May Allah guide you and set your affairs in order.
Yahdeekumul-laahu wa yuslihu baalakum.