رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 880
The Book of Greetings
Saying "Al-Hamdulillah" When Sneezing, Its Reply, and Manners Relating to Sneezing and Yawning
وَعَنْ أَبِي موسي رَضِيَ اللَّهُ عَنْهُ قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّه عَلَيْهِ وَسَلَّمَ يَقُولُ: إِذَا عَطَسَ أَحَدُكُمْ فَحَمِدَ اللَّهَ فَشَمِّتُوهُ، فَإِنْ لَمْ يَحْمَدِ اللَّهَ فَلاَ تُشَمِّتُوهُ ((رَوَاهُ مُسْلِم)).
Abu Musa (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "When one of you sneezes and praises Allah (i.e., says Al-hamdu lillah), you should invoke Allah's Mercy upon him (i.e., say Yarhamuk-Allah); but if he does not praise Allah, you should not respond." [Muslim].
Riyad as-Salihin, 880