What is the reason for the revelation of verses 18, 19, 20, and 21 of Surat Fussilat?

The verses mentioned from Surat Fussilat do not have a known specific reason for their revelation. The first verse clarifies the fate of previous nations that disbelieved their messengers, and Allah commanded the Prophet to warn idolaters of a similar torment. As for the other verses, they illustrate the punishment in the Hereafter and the testimony of body parts against their owners on the Day of Judgment, as a warning and intimidation to them, just as there followed a proof of the testimony of body parts in His (Allah's) saying: {And you were not accustomed to conceal yourselves lest your hearing, your sight, or your skins testify against you.} And Al-Shaykhan narrated from Ibn Mas'ud the story of the revelation of this verse.