Musannaf Ibn Abi Shayba 17:696

Ibn Fudayl narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu ‘Abd al-Rahman, from ‘Abdullah, who said: When one of you sneezes, let him say: "Praise be to Allah" (Al-hamdu lillah). And let those who are with him say: "May Allah have mercy upon you" (Yarhamuk Allah). And let him respond to them: "May Allah forgive us and you" (Yaghfir Allah lana wa lakum).