When a person sneezes, he should say اَلْحَمْدُ لله (all praise belongs to Allah Ta’ala).
The one who hears his sneeze and hears him saying Alhamdulillah, should reply to him by saying, يَرْحَمُكَ الله (may Allah Ta’ala have mercy on you).
In response, the one who sneezed should make dua for him saying: يَهْدِيكُمُ اللهُ وُيُصْلِحُ بَالَكُم (may Allah Ta’ala guide you in deen and dunya and improve your present condition in deen and dunya).
عن أبي هريرة رضي الله عنه، عن النبي صلى الله عليه وسلم قال: إذا عطس أحدكم فليقل: الحمد لله وليقل له أخوه أو صاحبه: يرحمك الله فإذا قال له يرحمك الله فليقل: يهديكم الله ويصلح بالكم (صحيح البخاري، الرقم: 6224)
Hazrat Abu Hurairah (radhiyallahu anhu) reports that Rasulullah (sallallahu alaihi wasallam) said, “When one of you sneezes, he should say, الْحَمْدُ لله. (In response), his brother or his companion should say, يَرْحَمُكَ الله. When he says يَرْحَمْكُ الله, the one who sneezed should make dua for him saying, يَهْدِيكُمُ اللهُ وُيُصْلِحُ بَالَكُم.”