What is the ruling on reciting the Tashahhud instead of Al-Fatiha in prayer, and what are the consequences of that?

If a worshipper forgets to recite Al-Fatiha and instead recites the Tashahhud while standing, there are two scenarios:

First: If he remembers before bowing (rukūʿ), he should recite Al-Fatiha, and his prayer is valid. It is recommended for him to perform the prostration of forgetfulness (sujūd al-sahw) for reciting a prescribed remembrance in an improper place.

Second: If he does not remember Al-Fatiha but recites the Tashahhud instead, then if he is an Imam or praying alone, the rak'ah is invalidated, and he must perform another rak'ah in its place. If a long time has passed, he must repeat the entire prayer. If he is a follower (ma'mūm), then it depends on the differing opinions regarding the ruling on the follower's recitation of Al-Fatiha. Those who consider it a pillar (rukn) would rule that the rak'ah is not counted, while those who say it is not obligatory for the follower would rule that there is nothing upon him.