Riyad as-Salihin, Book 6, Hadith 880 (In-Book) ⓘ
Abu Musa (May Allah be pleased with him) reported: The Messenger of Allah 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 make a response." [Muslim].
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" ((رواه مسلم)).