Musnad Ahmad 9:11

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Abd al-Malik ibn 'Umayr, he said: I heard 'Amr ibn Hurayth, he said: I heard Sa'id ibn Zayd saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The truffle is of the manna, and its water is a cure for the eye."