What is the ruling on the prayer of one who prostrated the second prostration in the second rak'ah, then stood up straight, then forgot and returned to prostrate again?

If a worshiper forgets the first Tashahhud and stands up, there are two scenarios:

1. If he remembers before starting the recitation: it is disliked for him to return, and some say it is forbidden.

2. If he remembers after starting the recitation: it is forbidden for him to return, by agreement of most scholars.

And if the worshiper returns to the first Tashahhud after having stood up completely:

1. If his return was with knowledge and intent: his prayer is invalidated.

2. If his return was due to ignorance or forgetfulness: his prayer is not invalidated.