Assalamualaikum Warahamathulahi Wabarakatuhu
Sneezing occurs when some disturbances occur in the nasal passage of nose. When one sneezes the vapors that were trapped would be released through the mouth, if they’re not released they would create some other problems in the body. And when we sneeze our whole shakes off (except heart), the whole body feels an earthquake. So, sneezing is a blessing, hence we thank the Almighty for giving it by saying ALHAMDHULILLAH.
And the one hearing it must say, “ Yarahmuk ALLAH”. It is said to rejoice over the misfortune of Shaytaan i.e.., when a mumeen (Believer) says ALHAMDHULILLAH after sneezing, it would annoy shaytaan, and that’s why the listener must say “Yarahmuk ALLAH”.
Narrated Abu Huraira ( Radhiallahu’Anhu):”The Prophet Muhammad (Salallahu’Alayhiwassalam) said, ” If anyone of you sneezes, he should say ‘Al-Hamduli l-lah’ (Praise be to Allah), and his (Muslim) brother or companion should say to him, ‘Yar-hamuka-l-lah’ (May Allah bestow his Mercy on you). When the latter says ‘Yar-hamuka-llah”, the former should say, ‘Yahdikumul-lah wa Yuslih balakum’ (May Allah give you guidance and improve your condition). “ [ Sahih Bukhari, Book 73, Vol 8, Hadith # 243]
And moreover it’s a Sunnah of the Prophet Muhammad (Salallahu’Alayhiwassalam). ALLAH likes sneezing, because it makes his servants to praise him “ALHAMDHULILLAH”. Hence, this is Why we say ALHAMDHULILLAH after sneezing.
JAZAKALLAH KHAIRUN [Courtesy Abdullah Alfazeena(Aleena Razee)]