What is the ruling on the prayer of someone who made a mistake in the remembrances (adhkar) that are obligatory in prayer, such as the remembrance while sitting between the two prostrations and the first tashahhud, out of forgetfulness or ignorance, and what is the ruling on the previous prayers in which this mistake occurred?

Firstly, the remembrance between the two prostrations (sajdatayn) is a Sunnah. Therefore, if one intentionally or ignorantly omits it, their prayer is not invalidated, and there is nothing upon them. However, if one forgets it, and it was their usual practice, then it is recommended for them to perform the prostration of forgetfulness (sujood as-sahw) before the salutation (tasleem).

Secondly, the first Tashahhud is obligatory. If one intentionally omits it, their prayer is invalidated. If one forgets it, they must perform the prostration of forgetfulness before the salutation. If one omits it out of ignorance, there is nothing upon them.