Musnad Ahmad 9:10

'Abd al-Rahman narrated to us, Sufyan narrated to us, from 'Abd al-Malik ibn 'Umayr, from 'Amr ibn Hurayth, from Sa'id ibn Zayd, who said: The Messenger of Allah, peace and blessings of Allah be upon him, came out to us, and in his hand was a truffle. He said: "Do you know what this is? This is from the manna, and its water is a cure for the eye."