Tafsir of Al-Fajr 89:4

It means: when it departs, similar to the words of the Exalted, “And the night when it retreats,” and “And the night when it departs.” It is apparent that this is a metaphorical expression or a metaphor, and the likeness is similar to the day. The word "when" (idha), according to what the scholar al-Taftazani explicitly stated in al-Talwih, is a substitute for the word "night," and its departure from being an adverb of time is acceptable. Alternatively, it is an adverb related to an implied noun, which is "grandeur," as chosen by some. The swearing by that time, or the restriction of that grandeur to it, is due to it providing clear evidence of perfect power and abundant blessings.

Or, it means "people travel in it," according to what Abu Hayyan narrated from al-Akhfash and Ibn Qutaybah, similar to the saying, "He prayed the station," meaning "he prayed in the station." This is a figurative attribution, attributing to the time what is attributed to the place. Regardless, the intention by "the night" is its genus. Mujahid, Ikrimah, and al-Kalbi said the intent is the night of the sacrifice (Laylat al-Nahr), when pilgrims travel in it to Muzdalifah after departing from Arafat, but this is not strong. The swearing or the restriction in the latter interpretation is due to the blessing of being protected from the heat of the sun and the evil of highway robbers, which is usually the case when traveling at night.

The letter ya is omitted by the majority in both recitation and pause from the end of yasri, even though it is the final radical of an imperfect verb that is not in the jussive mood. This is done to suffice with the kasra for the sake of lightness, and to ensure the verse endings correspond; this is why it is marked as such in the copies of the Quran (Mushafs). It should not be said that it was omitted because it dropped from the script, as that would necessitate that the reading follows the script rather than a prior transmission, which is incorrect. Nafi' and Abu 'Amr (in one narration) restricted this omission to the pause to observe the verse endings, while Ibn Kathir and Ya'qub did not omit it at all.

In the Tafsir of al-Baghawi, al-Akhfash was asked about the reason for the dropping of the ya in yasri, and he replied, "The night does not travel (yasri), but one travels in it (yusra fihi)." This is a rationalization often questioned due to its subtlety, and the answer is that when one deviates from the apparent meaning, the intention behind the meaning differs from the wording, for a thing draws its genus to its similarity, as it is said:

Birds only alight upon their own kind.

This is similar to what was said regarding the verse, "Your mother was never unchaste (baghiyya)," for when it deviated from baghiya, the ta was dropped, and it was not said baghiyyah. Such instances are among the marvels of the Arabic language. A similar explanation is possible regarding the interpretation of yasri as "it passes," due to the deviation from the apparent meaning, as it is known that it is a metaphor in that regard.

Abu al-Dinar al-A'rabi recited al-Fajr, al-Watr, and yasri with tanwin in all three. Ibn Khalawayh said: "This is as was narrated from some Arabs who would pause on the ends of rhymes with tanwin, even if they were verbs or contained the definite article al," such as the saying:

Lessen the blame, O censurer, and stop, And say, if I was right, that I have indeed hit the mark.

As Abu Hayyan mentioned, grammarians have noted this in absolute rhymes—meaning those that are vocalized—when the poet does not intone (taranum). This is one of two methods for the Arabs when they do not intone; the other is stopping (i.e., leaving the rhyme without tanwin), whereby they say al-'itab and asaba as they do when stopping on a word in prose. This Bedouin treated the verse endings as he would a pause and treated them like absolute rhymes. This tanwin is called tanwin al-taranum (the tanwin of intonation), and it is not exclusive to nouns. I am inclined to believe that it is written as a nun, unlike other types of tanwin which are exclusive to nouns.