Musnad Ahmad 1060:90

Hashim ibn al-Qasim narrated to us, Shu'bah narrated to us, from Muhammad ibn Abi Layla, from his brother, from his father, from Abu Ayyub, from the Prophet, peace and blessings of Allah be upon him, who said: "When one of you sneezes, let him say: 'Praise be to Allah in every state.' And let the one who says 'Yarhamuk Allah' (May Allah have mercy on you) to him say: 'Yarhamukum Allah' (May Allah have mercy on you all). And let the one who responds to him say: 'Yahdikum Allah wa yuslihu balakum' (May Allah guide you and rectify your state)."