What is the ruling on the prayer of one who did not fully straighten up after rising from bowing until after beginning the remembrance, then doubted his prostration after rising from it and returned to perform it without saying "Allahu Akbar"?

If the worshiper does not stand completely upright after rising from bowing, and remains slightly bent, their prayer is valid, as long as they do not reach the extent of bowing, because this does not negate their standing. As for someone who repeats prostration due to doubt about having omitted it, they should say 'Allahu Akbar' when repeating it, and their prayer is valid even if they omit this 'Takbir' according to the majority of scholars. However, the prayer becomes invalid if this 'Takbir' is intentionally omitted, according to the Hanbalis.