Tafsir of Ash-Shu`ara' 26:187

"Then cast down upon us fragments from the sky, if you are of the truthful."

This is a proposal that contains within it every form of denial, similar to the [verse]: "If this is indeed the truth from You... then rain down upon us stones from the sky." Perhaps they countered with this because of the implication of the command to fear God, as previously mentioned.

Kisafan (fragments) means pieces, as has been narrated from Ibn Abbas and Qatadah, the plural of kisafah, like qit‘ah (a piece). Most [reciters] read it as kisfan—with a kasra on the kaf and a sukun on the sin—which is also a plural of kisafah, like sidrah and sidr. It is said that kisaf and kisafah are like ri‘ and ri‘ah, meaning a piece.

"From the sky": The sky refers either to the canopy [above], which is the apparent meaning, or the clouds. It is apparent that the prepositional phrase [from the sky] is attached to a deleted [word] that serves as an adjective for what precedes it, and its attachment to asqit (cast down) is extremely weak. It is permissible to interpret the sky here as the direction of elevation.

The response to the in (if) is deleted, indicated by asqit (cast down). Those who permit the response to precede the conditional clause consider it to be the response.