Answered by Shaykh Faraz Rabbani
Does sajda sahw have to be made if an additional surah is recited after fatiha in the third or fourth rakah?
If one recites a sura after the Fatiha in the third or four rakats of an obligatory (fard) prayer, you have done something that is somewhat disliked (makruh tanzihan), and no forgetfulness prostration is needed, because it is only needed if a necessary (wajib) action is omitted.
However, in a non-obligatory (non-fard) prayer, it is necessary (wajib) to recite both the Fatiha and a sura (or the equivalent of 3 short verses) in all rakats of the prayer. If this is forgetfully left, a forgetfulness prostration is necessary. [Both rulings from Ibn Abidin’s Hashiya.]