Sayyiduna Salim bin Ubayd (may Allah be pleased with him) narrates: We were with the Prophet (peace and blessings be upon him) when a man sneezed and said: As-salamu alaykum (peace be upon you). The Prophet (peace and blessings be upon him) said: "And upon your mother too." Then he said: "When one of you sneezes, let him say: 'Al-hamdu lillahi rabbil-alamin kathiran' (abundant praise be to Allah, the Lord of the worlds), or: 'Al-hamdu lillahi ala kulli hal' (praise be to Allah in every state); and let the one who is with him say: 'Yarhamukallah' (may Allah have mercy on you); and let him reply to them: 'Yaghfirullahu li wa lakum' (may Allah forgive me and you)."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6339