Tafsir of Al-Muddathir 74:17

I will overwhelm him with a difficult, arduous obstacle of ascent. This is a metaphor for the grueling, difficult, and unbearable torment that he will encounter through the calamities and various hardships that God Almighty drives toward him. It likens his punishment to the task of climbing rugged, arduous mountains. The term is used here by way of representative metaphor (isti‘ara tamthiliyya).

It has been narrated by Ahmad, al-Tirmidhi, and al-Hakim—who, along with a group of scholars, classified it as authentic—on the authority of Abu Sa‘id al-Khudri in a marfu‘ (elevated) report: "Al-Sa‘ud is a mountain of fire; he is made to ascend it for a duration of seventy years, then he falls down it, and so on forever."

It is also narrated from the Prophet, may God Almighty bless him and grant him peace, that "he will be tasked with ascending an obstacle in the Fire. Every time he places his hand upon it, it melts, and when he lifts it, it returns to its state; and when he places his foot upon it, it melts, and when he lifts it, it returns to its state."