When one sneezes, he should say:
اَلۡحَمۡدُ لِلّٰهِ رَبِّ الۡعٰلَمِيۡنَ
His brother or his companion should say :
Then the person who sneezed, should say:
يَغْفِرُ اللهُ لِي وَلَكُمْ
– Sahih Al-Bukhari
Narrated AbuHurayrah (Radi Allahu Anhu): Respond three times to your brother when he sneezes, and if he sneezes more often, he has a cold in his head.