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 make a response." [Muslim].
App reference: Book 6, Hadith 36
Arabic/English book reference: Book 6, Hadith 880