Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) used to teach us: "When one of you sneezes, let him say: 'Alhamdu lillahi Rabbil-'alamin' (All praise is for Allah, the Lord of all the worlds). When he says that, let whoever is with him say: 'Yarhamuk Allah' (May Allah have mercy on you). When he says that, let the one who sneezed say: 'Yaghfirullahu li wa lakum' (May Allah forgive me and you)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10288