Sunan At-Tirmidhi

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)