Narrated Salim bin 'Ubaid:
that he was with some people on a journey, and a man among the people sneezed and he said: "As-Salamu Alaikum (peace be upon you)." So he (Salim) said: "'Alaika Wa 'Ala Ummik (upon you and upon your mother)." It seemed as if that bothered the man, so he said: "Indeed I have not said except what the Prophet (ﷺ) said; a man sneezed in the presence of the Prophet (ﷺ) and said: 'As-Salamu 'Alaikum (peace be upon you)' so the Prophet (ﷺ) said: ''Alaika Wa 'Ala Ummik (upon you and upon your mother). When one of you sneezes let him say: "Al-Hamdulillahi Rabbil-'Alamin (All praise is due to the Lord of all that exists)" and let the one responding to him say: Yarhamukallah (May Allah have mercy upon you)" and let him reply: Yaghfirullah Li Walakum (May Allah forgive me and you both)."
English reference: Vol. 5, Book 41, Hadith 2740
Arabic reference: Book 43, Hadith 2959