Is the prayer valid if one re-recites the Surah or repeats "Sami'a Allahu liman hamidah" (Allah hears him who praises Him) after the takbir for bowing or prostrating, and before reaching that same position, knowing that this occurs before interrupting the bowing or prostration?

The Sunnah for the transitional takbirat (uttering "Allahu Akbar") is to say them during the transition. Abu Hurairah reported: "When the Messenger of Allah, peace and blessings be upon him, stood for prayer, he would say takbir when he stood, then he would say takbir when he bowed, then he would say: 'Allahu sami'a li man hamidah' (Allah hears the one who praises Him) when he raised his back from the rak'ah, then while standing he would say: 'Rabbana wa lakal-hamd' (Our Lord, to You be praise), then he would say takbir when he descended to prostrate..." Scholars have concluded that the takbir should be said during the descent for bowing and prostration.

Ibn 'Uthaymin said: The takbir must be said between the start of the transition and its completion. The best approach is to begin the takbir during the descent and finish it before reaching the next pillar. However, there is no harm if one starts it before descending or completes it after reaching the position, and the prayer is not invalidated by this due to the difficulty involved.

If a worshipper says the takbir for bowing while standing and has not yet bowed, it is not considered as having started the bowing. Rather, it is a takbir uttered out of place while he is still standing. He may correct his recitation or the dhikr for bowing, then begin the takbir when he actually starts to bow or prostrate. Ibn Baz was asked about a man who said the takbir for bowing while standing, then remembered verses and completed them. He replied that there is nothing wrong with it because bowing is an action, not merely words.