Is sitting between the two prostrations in a manner resembling preparation for the next prostration acceptable, and is the prayer valid with this sitting?

One must sit between the two prostrations with straightness and ṭuma’nīnah (stillness/composure), as this is a pillar of prayer. This ṭuma’nīnah is estimated by the minimum stillness between the movements of rising and descending, or by the duration of a single tasbīḥah (recitation of 'Subhanallah'). Whoever sits straight to this extent, their prayer is valid. However, whoever fails to observe the obligatory ṭuma’nīnah, their prayer is invalid. There is a difference of opinion regarding the obligation of repeating the prayer if one was ignorant of the ruling, and the majority view is that repetition is obligatory.