وعن أبي موسي رضي الله عنه قال: سمعت
رسول الله صلى الله عليه وسلم يقول: «إذا عطس أحدكم فحمد الله فشمتوه، فإن لم يحمد الله فلا تشمتوه»
The Messenger of Allah (sallallahu alayhi wa sallam) 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.”.
Riyad as-Salihin 880, Book 6, The Book of Greetings, Hadith 37. 1 min read