Narrated Anas bin Malik: Two men sneezed before the Prophet. The Prophet said to one of them, "May Allah bestow His Mercy on you," but he did not say that to the other. On being asked (why), the Prophet said, "That one praised Allah (at the time of sneezing), while the other did not praise Allah."
Hadith Referenceصحيح البخاري / كتاب الأدب / 6221
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة
Hadith Commentary: (1) The one who sneezes becomes deserving of a response only after saying "alhamdulillah." A man said "alhamdulillah," so the Messenger of Allah (sallallahu alayhi wa sallam) responded to him, and another neglected to do so, so he (sallallahu alayhi wa sallam) did not respond to him.
(2) Sneezing is a cause of health, balance of temperament, and cleansing of the mind. For this, expressing gratitude to Allah—that is, saying "alhamdulillah"—is Sunnah, and it is incumbent upon the listener to respond to it. Thus, making supplication and informing him is necessary, because by fulfilling the obligations and rights, he has become deserving of this gift.
Source: Hidayat al-Qari: Commentary on Sahih Bukhari, Urdu, Page: 6221
´Narrated Anas bin Malik:` that two men sneezed in the presence of the Prophet (ﷺ); he responded to one of them and did not respond to the other. The one who was not responded to said: "O Messenger of Allah! You responded to this person and did not respond to me?' So the Messenger of Allah (ﷺ) said: "He praised Allah while you did not praise Him."
Shaykh Dr. Abdur Rahman Freywai
Explanation: 1: From this it is understood that only the one who says (Alhamdulillah) upon sneezing, according to the Sunnah, is deserving of the supplication of goodness. If he does not say (Alhamdulillah), then there is no need to respond. However, if the person sneezing is unaware of this ruling, he should be informed and made to understand.
Source: Sunan al-Tirmidhi – Majlis ‘Ilmi Dar al-Da‘wah, New Delhi Edition, Page: 2742