Is it permissible for a Muslim to participate in sessions for learning the rules of recitation (Tajweed) where attendees are asked to invoke blessings upon the Prophet 300 times silently before starting, or is it permissible for him to silently recite another form of remembrance, such as seeking forgiveness (Istighfar)?

Committing to a specific number of prayers upon the Prophet, peace and blessings be upon him, before a lesson is considered an innovation (bid'ah); as there is no evidence for it in the guidance of the Prophet, peace and blessings be upon him, nor his Companions. Worship must be legislated in its essence, manner, time, and quantity. Restricting remembrance (dhikr) to a specific number, time, or place without legal evidence makes it one of the innovated practices.

This is supported by what Al-Darimi narrated from Abdullah ibn Mas'ud, may Allah be pleased with him, regarding his condemnation of people who remembered Allah in circles with specific numbers that were not mentioned in the Sharia. Therefore, praying upon the Prophet, peace and blessings be upon him, is a great act of worship, but dedicating it to a specific number before every lesson is an innovated matter that has not been reported. It is obligatory to advise those who do this, and if they do not respond, one may cease learning from them to avoid the infiltration of innovation.