Musnad Ahmad 30:1510

Hujayn ibn al-Muthanna narrated to us, Abd al-Aziz narrated to us, from Abdullah ibn Dinar, from Abu Salih al-Samman, from Abu Hurairah, that the Prophet, peace and blessings of Allah be upon him, said: "If one of you sneezes, let him say: 'Al-hamdu lillah' (Praise be to Allah). If he says: 'Al-hamdu lillah', his brother should say to him: 'Yarhamuk Allah' (May Allah have mercy on you). If it is said to him: 'Yarhamuk Allah', let him say: 'Yahdikum Allah wa yuslih balakum' (May Allah guide you and rectify your state)."