Abu Dawud Book 36, Hadith Number 5013.
Narated By Salim ibn Ubayd : Hilal ibn Yasar said: We were with Salim ibn Ubayd when a man from among the people sneezed and said: Peace be upon you.
Salim said: And upon you and your mother. Later he said: Perhaps you found something (annoying) in what I said to you.
He said: I wished you would not mention my mother with good or evil. He said: I have just said to you what the Apostle of Allah (PBUH) said. We were in the presence of the Apostle of Allah (PBUH) when a man from among the people sneezed, saying: Peace be upon you!
The Apostle of Allah (PBUH) said: And upon you and your mother. He then said: When one of you sneezes, he should praise Allah. He further mentioned some attributes (of Allah), saying: The one who is with him should say to him: Allah have mercy on you, and he should reply to them: Allah forgive us and you.