What is the ruling on a Muslim reciting more than Surah Al-Fatihah in the third and fourth rak'ahs of prayer, knowing that the Messenger of Allah, peace and blessings be upon him, did not do so, and justifying his action by saying it is "better"?

Scholars have differed regarding the ruling on reciting a Surah in the third and fourth Rak'ahs. The preponderant and authoritative opinion, which is also the one adopted for issuing fatwas, is that of the majority of scholars, which states that recitation is not prescribed in the third and fourth Rak'ahs. This is based on the agreed-upon Hadith of Abu Qatadah. What this person did is contrary to the Sunnah.