Is a person considered an innovator (mubtadi') if they deviate from the Prophet (peace be upon him) in a simple matter related to their worship, such as changing the timing of an established act of worship, like supplicating in congregation after prayer or reciting Surat Al-Kahf on a day other than Friday?

A Muslim must follow the Prophet (peace be upon him) and avoid innovation (bid‘ah) in religion. Innovation is of two types: 1. Genuine innovation (bid‘ah haqiqiyyah): Inventing an act of worship that has no basis in Sharia, such as wiping over graves. 2. Additional innovation (bid‘ah idhafiyyah): The origin of the worship is prescribed, but the deviation occurs in specifying its time, place, number, manner, or reason. It becomes an innovation if done regularly and repeatedly. Examples include adhering to specific manners of dhikr (remembrance of Allah), or singling out specific acts of worship for times not legislated, such as fasting on the middle of Sha‘ban.

Supplication (du‘a) is prescribed during prayer and after it, but collective supplication is forbidden. The proof for the permissibility of supplication after prayer is what was narrated from the Prophet (peace be upon him) regarding the supplication "Allahumma ighfir li ma qaddamtu wa ma akhkhartu..." (O Allah, forgive me what I have sent forth and what I have held back...) after the taslim (salutation of peace), and the hadith: "Which supplication is most heard?" He said: "In the last part of the night and at the end of the obligatory prayers." And "at the end of the prayer" includes what comes after it. As for collective supplication after prayers, it has not been reported, and adhering to it is an innovation.

There is no harm in reciting Surat Al-Kahf on days other than Friday. The reciter will receive ten good deeds for every letter, provided that no specific day is singled out for it with a special virtue, as the mentioned superiority for its recitation is specific to Friday.