Musnad Ahmad 9:8

'Umar ibn 'Ubayd narrated to us, from 'Abd al-Malik ibn 'Umayr, from 'Amr ibn Hurayth, from Sa'id ibn Zayd, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "The truffle is of the manna, and its water is a cure for the eye."