Is it permissible to make a mistake in reciting the opening supplication (Du’a al-Istiftah) or to forget it, and is a prostration of forgetfulness (Sujud al-Sahw) required if one doubts having said “Sami’a Allahu liman hamidah” (Allah hears those who praise Him) after bowing, or “Rabbighfir li waliwalidayya” (My Lord, forgive me and my parents) between the two prostrations?

The opening supplication (Du'a al-Istiftah) is a recommended act in prayer. If someone omits it, or omits part of it, or makes a mistake in reciting it, nothing is incumbent upon them. The same applies to other recommended acts (sunan), such as saying "Sami'a Allahu liman hamidah" (Allah hears those who praise Him) after bowing, and the supplication between the two prostrations. The one who omits them is not required to perform the prostration of forgetfulness (sujud al-sahw).