Musnad Ahmad 9:3

'Abd al-Samad narrated to us, my father narrated to me, 'Ata' ibn al-Sa'ib narrated to us, from 'Amr ibn Hurayth, who said: My father narrated to me, from the Messenger of Allah, peace and blessings of Allah be upon him, that he said: "The truffle is of the manna, and its water is a cure for the eye."