What is the ruling concerning the prayer of an Imam who deliberately omits the first Tashahhud after the congregants remind him, and does this ruling apply to the prayer of an individual?

If a worshiper inadvertently stands for the third unit of prayer (rak'ah) without sitting for the first tashahhud (middle), and only remembers after having stood up completely, then he continues his prayer and performs two prostrations of forgetfulness (sujud al-sahw) before the salutation (taslim), whether he is an imam or praying alone. However, if he stood up completely and then intentionally returned, knowing that it is forbidden, scholars have differed regarding the invalidity of his prayer. Al-Shafi'i and one narration from Ahmad held the view that it invalidates the prayer, while Malik and the well-known narration from Ahmad held that it does not invalidate the prayer if he returns before starting the recitation. If the return occurs after he has begun the recitation, then returning is impermissible by scholarly consensus.