Does a Muslim sin and forfeit the reward for invoking blessings upon the Prophet (peace be upon him) or reciting the Quran if he is showing off, or is he rewarded for it out of veneration for the Prophet (peace be upon him) and glorification of the word of Allah Almighty? And is there any Sharia evidence to support this?

Good deeds are only accepted if they are purely for the sake of Allah the Almighty. If they are tainted with ostentation (riya'), they become null and void and are rejected. Ostentation is divided into categories: First: Pure ostentation, where the sole intention is to show off to creation for a worldly purpose. This action is null and void, and its doer deserves punishment. Second: When an action is for Allah, but ostentation is mixed with it. If it is mixed with its origin, the action becomes void and nullified. If the origin of the action was for Allah, and then the intention of ostentation occurred to him, if it was merely a fleeting thought that he pushed away, it does not harm him. But if he indulged in it, scholars have differed on whether his action becomes void. The more preponderant view is that it does not become void, and he is rewarded for his initial intention. This difference of opinion is specific to actions where the end is connected to the beginning, like prayer. As for actions where there is no connection, like recitation, it is interrupted by the intention of ostentation. Third: If a person performs an action purely for Allah, and then Allah casts good praise for him in the hearts of the believers, and he rejoices at that, it does not harm him. That is the immediate good tidings for the believer. Sincerity is precious and difficult for the soul, and the servant must continually check his sincerity and hold himself accountable.