Musnad Ahmad 9:1

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