Tafsir of Al-Inshiqaq 84:23

"And Allah is most knowing of what they keep in containers."

That is, [He is most knowing] of that which they harbor within their chests—of disbelief, envy, and hatred. Ma (what) is a relative pronoun (mausulah), and the referential pronoun (al-'a'id) is omitted.

The origin of al-i'ad (to store/keep in a container) is the placing of something into a vessel (wi'a). In Mufradat al-Raghib, it is stated that al-i'a' is the preservation of provisions in a vessel, from which is the saying:

And evil is the foulest of provisions you store.

Here, it is used metaphorically to mean "harboring" [thoughts or secrets]. This is what has been narrated from Ibn Abbas. It does not necessitate that the verse refers specifically to the hypocrites despite the Surah being Meccan, as is evident.

Some interpreted it as "gathering," as reported from Ibn Zayd. It is also permissible that the meaning is: "And Allah the Exalted is most knowing of that which they gather in their scrolls of evil deeds." Regardless, Allah’s knowledge of that is a metonymy for His, glory be to Him, recompensing them for it.

It is also said that the intent is to indicate that behind their rejection lie many great evils, the extent of which exceeds the scope of expression. Others have said it is possible that the meaning is: "And Allah the Exalted is most knowing of what they harbor within themselves of the proofs that he [the Prophet] is true, meaning the Quran is the truth." Thus, the intent would be to emphasize their stubbornness and their denial contrary to their own knowledge. It is apparent that, according to this [interpretation], the sentence is a circumstantial clause (hal) describing the pronoun in "they deny" (yukathibun). To consider it otherwise, based on what was said regarding it being an indication [of hidden evils], is contrary to the apparent meaning.

Abu Raja’ recited: "bima ya'un" (by what they keep in memory/guard), from the root wa'a-ya'i (to keep/memorize).