How can we reconcile the impermissibility of innovation in religion with the action of the Companion who added to the remembrance (dhikr) after rising from bowing, and which the Prophet (peace be upon him) approved?

The Sunnah can be a saying, an action, or a tacit approval. The Companion's praise was a permissible form of praise during prayer, and the Prophet (peace and blessings be upon him) approved it, making it a Sunnah and a regular remembrance with this wording. This has been used as evidence for the permissibility of introducing a new remembrance in prayer, even if not explicitly narrated, as long as it does not contradict what has been narrated.