Musnad Abu Ya'la al-Mawsili 41:20

Shayban narrated to us, Jarir ibn Hazim narrated to us, Abd al-Malik ibn Umayr narrated to us, Amr ibn Hurayth narrated to us, from Sa'id ibn Zayd, that the Messenger of Allah (may Allah bless him and grant him peace) was asked about truffles. He said: "They are of the manna, and their water is a cure for the eye."