Tafsir of Qaf 50:40

"And from the night" is an object for a deleted verb explained by the phrase "glorify Him," considering their generic unity, while the conjunction denotes difference in the specific instance. This means: "Glorify Him during a part of the night; so glorify Him." Alternatively, it is an object for His saying "Glorify Him," on the basis that the fa is retributive, with the implied meaning: "Whatever may happen, glorify Him during a part of the night." The object is placed first to emphasize it, to serve as a compensation for the deleted [verb], and to allow the retributive fa to be in its rightful middle position. Perhaps what is intended by this "part" is the time before dawn (al-sahar), as its excellence is well-known.

"And at the posteriors of the prostrations" (wa adbār al-sujūd): This refers to the ends of the prayers. It is the plural of dubr (with a damma followed by a sukūn) or dubr (with two dammas). Ibn Abbas, Abu Ja‘far, Shaybah, ‘Isa, al-A‘mash, Talhah, Shibl, and the two Meccan/Medinan readers recited it as idbār with a kasrah on the alif; it is a verbal noun, as you say: "The prayer adbarat idbāran," meaning it ended and was completed. The meaning is: "And at the time of the conclusion of the prostration," similar to their saying, "I will come to you at the setting of the stars."

Many scholars hold that "glorification" (tasbih) refers to prayer, interpreting it as the mention of a part to signify the whole, or the signifier for the signified. On this basis, "before the sunrise" is the Dawn prayer, and "before the sunset" is the Afternoon (‘Asr) prayer—this was stated by Qatadah, Ibn Zayd, and the majority. Al-Tabarani in al-Awsat and Ibn ‘Asakir narrated from Jarir ibn ‘Abdullah as a marfu‘ hadith: "And from the night" is the Night (‘Atama) prayer, and "the posteriors of the prostrations" are the supererogatory prayers after the obligatory ones; Ibn Jarir narrated this from Ibn Zayd.

Ibn Abbas said: "Before the sunrise" is the Dawn prayer, "before the sunset" are the Noon (Zuhr) and Afternoon (‘Asr) prayers, "from the night" are the two Night prayers (Maghrib and ‘Isha), and "the posteriors of the prostrations" are the supererogatory prayers after the obligatory ones. In another narration from him, it is the Witr prayer after ‘Isha. In yet another narration from him, as well as from ‘Umar, ‘Ali, his son al-Hasan, Abu Hurayrah (may Allah be pleased with them all), al-Sha‘bi, Ibrahim, Mujahid, and al-Awza‘i, it is the two rak‘ahs after Maghrib. Musaddad narrated this in his Musnad, as did Ibn al-Mundhir and Ibn Mardawayh from ‘Ali (may Allah honor his face) as a marfu‘ hadith.

Muqatil said: "It is two rak‘ahs after ‘Isha, in which one recites 'Say, O disbelievers' in the first and 'Say, He is Allah, One' in the second." It was also said: "From the night" refers to the two Night prayers and the Tahajjud. From Mujahid, it is the night prayer. In this, there is a possibility of it being general for both Night prayers or specific to the Tahajjud, and the latter is the more evident interpretation.