Musnad Ahmad 5:409

'Abdullah narrated to us, Dawud ibn 'Amr al-Dabbi narrated to us, Mansur ibn Abi al-Aswad narrated to us, from Ibn Abi Layla, from al-Hakam, or 'Isa—Mansur was uncertain—from 'Abd al-Rahman ibn Abi Layla, from 'Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you sneezes, let him say: 'Praise be to Allah in every state.' And let the one who is with him say to him: 'May Allah have mercy on you.' And let him reply to them: 'May Allah guide you and rectify your state.'"