Is it obligatory for the worshipper to perform the prostration of forgetfulness (sujud al-sahw) if, while reciting a surah, he remembers that he did not recite Al-Fatihah and then recites it, or if he doubts whether he recited Al-Fatihah while reciting a surah and then recites it?

If a worshipper recites a Surah before Al-Fatiha by mistake, then remembers and recites Al-Fatiha, there is nothing upon him. What he recited before it is not counted, and he is not required to perform the prostration of forgetfulness (sujud sahwi), because what he did intentionally does not invalidate the prayer.

As for one who doubts whether he recited Al-Fatiha, the default is that he has not recited it, so he must recite it, and he is not required to perform the prostration of forgetfulness, because reciting Al-Fatiha at that point means he has performed the pillar in its proper place. The additional recitation is considered a permissible remembrance that is out of its proper place, which does not necessitate the prostration of forgetfulness.