It is narrated from Ka'b al-Ahbar that a man took off his shoes. Ka'b al-Ahbar said: Why did you take off your shoes? Perhaps you took them off after seeing this verse, when Allah, the Glorious and Exalted, said to Musa (peace be upon him) as he was going to the Mount: «﴿فَاخْلَعْ نَعْلَيْكَ﴾» "Take off your shoes." But do you know what Musa's (peace be upon him) shoes were made of?
Imam Malik (may Allah have mercy on him) said: I do not know what the man replied. Ka'b said: The shoes of Musa (peace be upon him) were made from the skin of a dead donkey.