Truffles are a kind of 'Manna' which Allah sent down upon Moses and their juice is a medicine for the eyes. (Using translation from Muslim 2049e)  

عَنِ النَّبِيِّ ﷺ قَالَ «الْكَمْأَةُ مِنَ الْمَنِّ الَّذِي أَنْزَلَهُ اللهُ عَلَى مُوسَى وَمَاؤُهَا شِفَاءٌ لِلْعَيْنِ»  

Messenger of Allah ﷺ said, "The Kam'a (i.e. a kind of edible fungus) is like the Manna (in that it is obtained without effort) and its water is a (medicine) cure for eye trouble." (Using translation from Bukhārī 4478)  

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «الْكَمْأَةُ مِنَ الْمَنِّ وَمَاؤُهَا شِفَاءٌ لِلْعَيْنِ»