Answered by Shaykh Faraz Rabbani
In obligatory prayers, if in the 3rd and/or 4th rakat we forgetfully DID recite a second surah, do we perform sajdatu sahw, redo our salah, or neither?
Walaikum assalam wa rahmatullah,
In the Name of Allah, Most Merciful and Compassionate. May His abundant blessing and most perfect of peace be on His Beloved Prophet, the best of creation, and his family, companions and followers.
Nothing: reciting after the Fatiha in the final 2 rakats of the obligatory prayer is somewhat disliked (makruh tanzihan), entailing impropriety but not anything that requires repetition or prostrations. [Ibn Abidin, Radd al-Muhtar]