Fussilat: (13) Then if they turn away, say...

(Then if they turn away) is connected to His, the Exalted’s, saying: "(Say, 'Do you indeed...')" and so on. That is: If they turn away from reflecting upon what has been mentioned of the momentous matters that call to faith, or from faith after this clarification, (then say) to them: (I have warned you) that is, I warn you. The past tense form is used to indicate the certainty of the warning, which portends the certainty of the warned-upon event, (a thunderbolt like the thunderbolt of 'Ad and Thamud).

(That is, their punishment is like their punishment. Qatadah said this, and it is apparent according to the view that the term "thunderbolt" (sa'iqah) comes in the language to mean "punishment." Some have disallowed this, treating what was mentioned as metaphorical, and that the meaning is "a punishment of severe impact, as if it were a thunderbolt like their thunderbolt." Regardless, the intent is: I have informed you of the occurrence of a thunderbolt.

Ibn al-Zubayr, al-Sulami, and Ibn Muḥayṣin read it as (ṣa'qatan mithla ṣa'qatihim) without an alif in both instances and with a quiescent 'ayn. It is the noun of instance (marrah) from al-ṣa'q or al-ṣa'iq. It is said: "The thunderbolt struck him (ṣa'aqat-hu al-ṣa'iqah), a striking (ṣa'qan), so he perished (fa-ṣa'aqa)—with a fathah—meaning he perished by the thunderbolt that struck him."