Tafsir of Al-Muzzammil 73:18

The Heaven is cleft means split. It has also been recited as munfatir (cleft), which implies mushtaqq (split).

Thereby, meaning by that Day and that event. The 'ba' (in bihi) is for the instrument, similar to your saying, "I split the stick with the axe," so it split by the axe. This means that the heaven, despite its greatness and precision, splits due to the severity and terror of that Day, just as something is split by that which causes it to split. What then would you think of the other created beings?

It is permissible that it is intended that the heaven is weighed down by it now, a weighing down that leads to its splitting due to its magnitude and its fear of its occurrence, similar to His saying: "It will be heavy in the heavens" (al-A'raf: 187). Thus, the speech is of the metaphorical kind, and the splitting is a metonymy for exaggeration regarding the weight of that Day. The intent is to convey that it is currently in this state. The first interpretation is more manifest and aligns better with the majority of the verses.

The apparent construction would have been "the heaven is cleft" (munfatirah) by feminizing the predicate, because it is well-known that "heaven" (sama') is feminine. The masculine form was used to indicate that the description is applied to a masculine noun, meaning "a thing that is cleft thereby." The subtle point in this is to alert that its true essence has changed, its name and character have ceased, and nothing remains of it except what can be referred to as "a thing."

Abu Amr ibn al-Ala, Abu Ubaidah, and al-Kisa'i—followed by Mundhir ibn Sa'id—stated that the masculine form is due to the interpretation of the heaven as a "ceiling" (saqf). The point of this is to evoke the meaning of "ceiling-ness" and "shading," so that the matter of the splitting becomes more staggering and horrific.

Al-Farisi said the estimation is "a possessor of splitting" (dhat infitar), similar to their saying "a nursing woman" (imra'ah murdi'), meaning "a possessor of nursing," thus it follows the way of attribution. It is also narrated from him that this is of the category of "spread locusts," "green trees," and "uprooted palm trunks"—meaning that "heaven" is of the category of a collective noun (ism jins) between which and its singular form is the feminine 'ta'. Its singular is sama'ah, and for a collective noun, both masculine and feminine are permissible; thus, it came as munfatir in the masculine.

Al-Farra said "heaven" implies "the canopy" (mazallah), and it is also feminine; thus, it came as munfatir in the masculine. From this is the poet's saying: Had he raised the heaven towards them... We would have reached the heaven and the clouds. On this basis, there is no need for interpretation, and one should only seek the subtle point for considering the masculine form when the majority of usage considers the feminine. Perhaps it is apparent to anyone with the slightest understanding.

Carrying the 'ba' in bihi to mean the instrument is more in harmony with terrorizing the matter of that Day. It is also permissible to carry it to denote containment (adverbial), meaning "the heaven is cleft in it." The return of the pronoun in the genitive case to "the Day" is the manifest view held by the majority.

Mujahid said it returns to Allah the Almighty, meaning by His command and His authority—Exalted is He. In his view, it is like the pronoun in His saying: "His promise is a thing to be fulfilled" (al-Muzzammil: 18), for it belongs to Him, the Almighty, based on what is known from the context, with the verbal noun added to its agent. It is also possible that it refers to the Day, like the pronoun in bihi according to the majority, with the verbal noun added to its object.