Muwatta Malik 1767

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 sneezed and someone said to him, "May Allah have mercy on you," (Yarhamuka'llah), he said, "May Allah have mercy on us and you, and forgive us and you." (Yarhamuna'llah wa yaghfirlana wa lakum)

Reference

Muwatta Malik 1767

In-book

Book 54, Hadith 5