Malik related to me from Nafi that when Abdullah ibn Umar sneezedand someone said to him, "May Allah have mercy on you,"(Yarhamuka'llah), he said, "May Allah have mercy on us and you, andforgive us and you." (Yarhamuna'llah wa yaghfirlana wa lakum).
USC-MSA web (English) reference: Book 54, Hadith 5
Arabic reference: Book 54, Hadith 1770