Tafsir of Al-A'raf 7:170

{And those who hold fast to the Book...}

There are two interpretations regarding this:

1. It is in the nominative case (marfūʿ) as an initial subject (mubtadaʾ), and its predicate is {Indeed, We do not allow to be lost the reward of the reformers}. The meaning is: We do not allow their reward to be lost, because "the reformers" carries the same meaning as "those who hold fast to the Book," similar to His saying: {Indeed, those who have believed and done righteous deeds - indeed, We do not allow to be lost the reward of any who did well in deeds} (Al-Kahf: 30).
2. It is in the genitive case (majrūr) as a conjunction to {those who fear [Allah]}, and the statement {Indeed, We do not allow to be lost...} is a parenthetical clause.

It has been recited as yumassikūna (with a shaddah). This is supported by the reading of Ubayy: {And those who have held fast (masakū) to the Book}.

If you ask: Holding fast to the Book encompasses every act of worship, including the establishment of prayer, so why was prayer singled out?

I say: It is to highlight the merit of prayer, as it is the pillar of the religion and the separator between disbelief and faith.

Ibn Masʿūd (may Allah be pleased with him) recited: {And those who have grasped firmly (istamsakū) to the Book}.

{And [mention] when We raised the mountain above them as if it were a canopy, and they were certain that it would fall upon them, [and We said], "Take what We have given you with determination and remember what is in it that you might become righteous."}