What is the evidence for the innovation (bid‘ah) of saying “Taqabbal Allah” (May Allah accept [it]) after finishing the prayer, and who are the scholars who issued fatwas to that effect, and why is it considered an innovation even though it is a supplication (du‘aa’)?

The fundamental principle in acts of worship is adherence [to prophetic tradition]. Thus, whatever has been authentically reported from the Prophet (peace be upon him) and his companions is a Sunnah (prophetic tradition), and whatever has not been reported is an innovation (bid'ah). Shaking hands and saying "Taqabbal Allah" (May Allah accept [your worship]) immediately after finishing the prayer has no basis in the Prophetic Sunnah or in the actions of the Companions. Sheikh Ibn 'Uthaymeen was asked about this and he stated that it has no basis. It is important to note that wisdom and gentleness should be employed when clarifying such matters, and one should emulate the Companions in teaching people the Sunnahs.