"They shall have coverings of fire from above them and coverings from beneath them" is like His saying: "They shall have a bed of Hell, and over them coverings (of fire)."

"That" is what "Allah frightens His servants with, so O My servants, fear Me."

Muhammad said: His saying "That is what Allah frightens His servants with," the word "That" is in the nominative case as an initial subject. The meaning is: That which we have mentioned of the punishment is what Allah uses to frighten His servants. As for His saying "O My servants," it is the recitation of Nafi' with the omission of the [ya] (the letter 'y' indicating possession); and this is the preferred reading according to the masters of Arabic linguistics.

Tafsir of Surat al-Zumar from verse 17 to verse 21.