Does repeating some of the words of the prayer intentionally or out of forgetfulness, or speaking during the prayer with something that does not invalidate it, necessitate the prostration of forgetfulness (sujud al-sahw)? And what is the difference between what invalidates the prayer if done intentionally and what does not invalidate the prayer if done intentionally, concerning the prostration of forgetfulness?

Scholars have divided acts that invalidate prayer, whether done intentionally or unintentionally, into two categories:

First: Acts that do not invalidate prayer if done intentionally, such as turning one's head, placing a hand over the mouth, thinking during prayer, looking at something distracting, and similar actions. For these, there is no prostration for intentionally doing them, nor for doing them out of forgetfulness.

Second: Acts that invalidate prayer if done intentionally, such as speaking, or adding extra bows (ruku') or prostrations (sujud). For these, one prostrates for forgetfulness, as long as the prayer has not been invalidated by them. However, if the prayer has been invalidated by them, there is no prostration, such as eating, excessive movement, or excessive speech done out of forgetfulness. Similarly, if a ritual impurity (hadath) occurs, even if due to forgetfulness, there is no prostration.

Main conclusion:

If one recites some of the prescribed remembrances (adhkar) of the prayer in the wrong place, it is commendable to perform the prostration of forgetfulness. However, if one repeats the Fatiha or the Tashahhud in its designated place, prostration for this is considered a possibility among the Shafi'is, and the relied-upon opinion among their later verifying scholars is that prostration is required, whether it was done intentionally or out of forgetfulness.