الكمأة
A desert fungus described as a form of manna, with its liquid recommended as an eye treatment.
الْكَمْأَةُ مِنَ الْمَنِّ، وَمَاؤُهَا شِفَاءٌ لِلْعَيْنِ
“Truffles are of the manna [given to the Israelites], and their water is a healing for the eye.” (meaning, not a literal translation)
"Manna" (al-mann) refers to the effortless sustenance described in the Qur'an as having been provided to the Israelites without cultivation — the comparison to truffles reflects that they, too, grow without being planted or tended. The hadith specifically names the liquid within or extracted from the truffle, not the flesh itself, as the part beneficial for the eye.
Ibn al-Qayyim discusses truffle liquid mixed with kohl as an eye treatment, while noting scholars differ on whether the benefit applies broadly or was specific to the truffle varieties known in Hijaz. He records that some scholars understood the "water" to mean the moisture naturally present within a fresh truffle, while others describe a preparation made by pressing or cooking it, and discusses reports of its use for eye inflammation and general irritation.
Desert truffles have been studied in modern food-science literature for antioxidant and anti-inflammatory compounds. Noted here as separate modern context, not as scientific validation of the hadith's specific eye-treatment claim.