1. When you sneeze, then say Alhamdu illah ('all praises and thanks are to Allah')
2. Your companion should then say Yarhamukallaah" ('may Allah have mercy upon you').
3. When someone says Yarhamukallah you should then say Yahdeemukul-laahu wa yuslihu baalakum ('may Allah guide you and set your affairs in order').
If an unbeliever sneezes and praises god, you should only say the last part.
Bukhari 7/125, Ahmad, Tirmirdhi, Dawood
[Fortress of the Muslim: Invocations from the qur'an & sunnah; Darussalam]
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