Tafsir of Al-Mu'minoon 23:9

"And those who are regarding their prayers"—the ones obligatory upon them, as Ibn al-Mundhir recorded from Abu Salih, and ‘Abd ibn Hamid from ‘Ikrimah—"maintaining."

[This means] by performing them in their proper times, fulfilling their conditions, and completing their bowing (ruku‘), prostration (sujud), and all their other essential components (arkan), as was narrated from Qatadah.

A group [of scholars] recorded from Ibn Mas‘ud that it was said to him: "Verily, Allah the Exalted frequently mentions prayer in the Quran—'[Those] who are regarding their prayers ever-constant' and '[Those] who are regarding their prayers maintaining'—what is intended?" He replied: "That is regarding their fixed times." They said: "We did not think this meant anything other than performing them and not abandoning them." He replied: "Abandoning them is disbelief (kufr)."

It has been said: Maintaining them means persistence in performing them in the most perfect manner. The verbal form (fi‘l) was used instead of the nominal form (ism), just as in the other preceding verse endings, because of the inherent renewal and repetition involved in prayer. For this reason, it is pluralized in the reading of the seven [canonical reciters], with the exception of the two brothers (Hamzah and Al-Kisa’i).

This is not a repetition of what was previously described regarding their submissiveness (khushu‘) in the general act of prayer, due to the total distinction between what is mentioned here and what was mentioned there, as is not hidden.

In beginning the attributes [of the believers] with prayer and concluding them with it is an exaltation of its status. The prioritization of khushu‘ (submissiveness) is due to its importance, for prayer without it is as if there were no prayer at all, by consensus. Indeed, they have said: "Prayer without khushu‘ is a body without a soul." It has also been said that its prioritization is due to the generality of what is meant by it here.