Is the aforementioned message, which includes an account of the reward based on the Prophetic Hadith "Prayer in congregation is twenty-seven degrees more virtuous than prayer alone," considered a specification of the reward, and is there any sharia prohibition in this specification?

The determination of reward ( اجر ) is a matter of divine prerogative ( توقيفي ), not ascertainable through independent reasoning ( اجتهاد ), and it is impermissible to claim a specific reward for an act of worship without evidence. What the Shari'ah has explicitly determined, such as the reward for congregational prayer, Duha prayer, and night prayer (Qiyam al-Layl), is not objectionable. Scholars have considered a Companion's statement regarding the determination of reward as having the ruling of a prophetic tradition (marfu' hukman), because there is no room for personal opinion in such matters.