What is the ruling on reciting a surah after Al-Fatiha in prayer when praying with another person? And is that considered showing off (riya')? And what should be done if it is so?

Reciting a surah after Al-Fatiha in prayer is a Sunnah that should not be neglected, in emulation of the Prophet, peace and blessings be upon him, and his saying: "Pray as you have seen me praying." If a worshiper confines himself to Al-Fatiha alone when praying individually, but adds a surah when leading people in prayer, his action is judged according to his intention: 1. If the intention is to show perfection to people, then it is forbidden ostentation (riya') and minor shirk, and the doer will be held accountable for it. 2. However, if the motivation is activity due to the congregation or advising the congregants to complete the prayer with its Sunnah acts, then this is a praiseworthy intention and he is hoped to be rewarded for it.

Anyone who acts in such a pretentious and ostentatious manner should repent, regret, and resolve not to return to it.