Answered by Shaykh Faraz Rabbani
I f one recites a surah after the fatihah in the 3rd or 4th rakah of a fard salat, would this entail sajda as- sahw [forgetfulness prostrations]?
1. Forgetfulness prostrations would not be required if one adds a surah (or a few verses) after the Fatiha in the 3rd or 4th rakat of an obligatory (fard) prayer.
2. Doing so is somewhat disliked in obligatory prayers. [Durr al-Mukhtar & Radd al-Muhtar]
3. Note, of course, that it is necessary (wajib) to recite both the Fatiha and the equivalent of 3 short verses in all rakats of all necessary (wajib) and non-obligatory (nafl) prayers. Leaving this out does necessitate forgetfulness prostrations.
4. Forgetfulness prostrations are necessary only when a necessary (wajib) act is omitted or unduly delayed forgetfully.
5. When a necessary act is left intentionally it is a sin: one must repent, and it is necessary to repeat the prayer within the prayer time.
6. When something prohibitively disliked (makruh tahriman) is done in the prayer, the prayer remains valid but is grossly deficient. As such, it is necessary (wajib) to repeat the prayer within the prayer time. [Durr & Radd al-Muhtar]