Musnad Abu Ya'la al-Mawsili 41:18

Al-Qawariri narrated to us, Ibn 'Uyaynah narrated to us, from 'Abd al-Malik, from 'Amr ibn Hurayth, from Sa'id ibn Zayd, from the Prophet, may Allah bless him and grant him peace, who said: "The truffle is of the manna which was sent down upon the Children of Israel, and its water is a cure for the eye."