Shaykh ul-Islaam Ibn Taymeeyah said:

❝So we have the one who prayed badly, he was ignorant. If the obligation of being tranquil and calm is known during within the time of the prayer then being tranquil and calm becomes obligatory upon him at that time, and it was not obligatory upon him before that time.❞

[Majmoo Al-Fatawa, (22/44) | Translated By Abbas Abu Yahya Miraath Al-Anbiyya]