وَحَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ إِذَا عَطَسَ فَقِيلَ لَهُ يَرْحَمُكَ اللَّهُ ‏.‏ قَالَ يَرْحَمُنَا اللَّهُ وَإِيَّاكُمْ وَيَغْفِرُ لَنَا وَلَكُمْ ‏.‏

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). Translation Not Available

Book reference: Muwatta Imam Malik Book 54 Hadith 1770
Web reference: Muwatta Imam Malik Book 54 Hadith 5