Riyad as-Salihin
Riyad as-Salihin 881
Saying "Al-Hamdulillah" When Sneezing, Its Reply, and Manners Relating to Sneezing and Yawning
Arabic hadith text
وعن أنس رضي الله عنه قال: عطس رجلان عند النبي صلى الله عليه وسلم ، فشمت أحدهما ولم يشمت الآخر، فقال الذي لم يشمته: عطس فلان فشمته، وعطست فلم تشمتني؟ فقال: "هذا حمد الله، وإنك لم تحمد الله" ((متفق عليه))
Translation
Anas (May Allah be pleased with him) reported
When two men sneezed in the presence of the Prophet (ﷺ), he responded to one with "Yarhamuk-Allah (may Allah have mercy on you)" and did not respond to the other. The latter said to him: "You invoked a blessing on this man but did not do so in my case." The Prophet (ﷺ) replied, "He praised Allah (i.e., he said 'Al-hamdu lillah') but you did not."[Al-Bukhari and Muslim].