Is repeating the phrase "Sami'a Allahu liman hamidah" (Allah hears him who praises Him) after rising from Ruku' considered a mistake, and is it obligatory to repeat the two prayers in which this occurred?

The saying "Sami'a Allahu liman hamidah" (Allah hears the one who praises Him) is a Sunnah (recommended act) according to the majority of scholars, and prayer is not invalidated by its omission. However, it is an obligatory act according to the Hanbalis, and prayer is invalidated by its deliberate omission.

If the worshipper pronounces it incorrectly, it is legislated for him to repeat it. As for past prayers in which the worshipper doubts the correctness of this remembrance, they are valid, even according to the view that it is obligatory, because if an obligatory act is unintentionally omitted and the prostration of forgetfulness (Sujud al-Sahw) is missed, the prayer is still valid and does not need to be repeated.