Tafsir of Al-Anbiya' 21:106

(Indeed in this)—that is, in what has been mentioned in this noble Surah of reports, profound admonitions, promises and threats, and definitive proofs indicating [Divine] Unity and the validity of Prophethood. It has also been said that the reference is to the entire Qur’an—(is a sufficient communication)—meaning a sufficiency, a means of attaining one’s goal, or the attainment itself, by way of hyperbole—(for a people who are worshippers).

(Meaning: for a people whose primary concern is worship rather than habit.) Ibn Abi Hatim recorded from al-Hasan that they are those who perform the five daily prayers in congregation. Ibn Marduyah recorded from Ibn Abbas that the Messenger of Allah (may Allah exalt his mention and grant him peace) recited this and said: "It is the five prayers in the Sacred Mosque in congregation." The pronoun "it" refers to the worship understood from "worshippers." Abu Hurairah, Muhammad ibn Ka'b, and Mujahid said: "It is the five daily prayers," without qualifying them with anything further. From Ka'b al-Ahbar, it is interpreted as the fasting of the month of Ramadan and the five prayers. The apparent meaning is generality, and that what has been mentioned is by way of restricting [the mention] to some individuals for a specific purpose.