What is the evidence from the Qur’an, the Sunnah, or the saying of a Companion that stipulates that the Khuff (leather sock) must be thick or opaque?

The Hanbali jurists — who stipulate that socks must be thick enough to walk in — argue that the permissibility of wiping over leather socks (khuffayn) came about due to the necessity of walking in them. Therefore, anything that shares their meaning, such as thick socks, is analogized to them. As for anything that cannot be continuously walked in, there is no need for it, and thus, wiping over it is not permissible.

Shaykh al-Islam Ibn Taymiyyah explained that the reason for which wiping over leather socks was permitted is the customary walking in them. This applies to thick socks that are customarily walked in, not to thin ones that tear quickly or do not stay on by themselves. He cited as evidence the Prophet's (peace be upon him) wiping over leather slippers (muqayn), socks (jawrabayn), and sandals (na'layn), and that wiping over socks was widespread among the Companions.