Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
2- However, if one – intentionally or unintentionally – adds another Surah, it does not necessitate a prostration of forgetfulness (Sujud al-Sahw).
In his answer to your question, Mufti Muhammad Ibn Adam Al-Kawthari, Director and researcher at the Institute of Islamic Jurisprudence (Darul Iftaa, www.daruliftaa.com), Member of the Al-Qalam Shari`ah Scholars Panel, and advisor on Islamic Banking, states:
According to the Hanafi School of Islamic law, it is not impermissible or prohibitively disliked to add another Surah after reciting Surat al-Fatihah in the third or fourth rak`ahs of an obligatory prayer; rather, it is somewhat disliked and ‘better’ to avoid, since the Sunna is to suffice with Surat al-Fatihah.
Abu Qatadah (may Allah be pleased with him) relates that the Messenger of Allah (peace be upon him) would recite in the first two rak`ahs the Umm al-Kitab [i.e. Surat al-Fatihah] and two Surahs, and in the last two the Umm al-Kitab [on its own]…” (Al-Bukhari)
As such, it is better to simply recite Surat al-Fatihah in the third and fourth rak`ahs of obligatory prayers. However, if one – intentionally or unintentionally – adds another Surah, it does not necessitate a prostration of forgetfulness (Sujud al-Sahw).
It is stated in Al-Fatawa al-Hindiyya, a well-known Hanafi Fiqh reference work, “If one recites in the last two rak`ahs [of a four unit obligatory prayer] Surat al-Fatihah and another Surah, a Sujud of forgetfulness (sahw) will not be necessary. This is the more correct position.” (Al-Fatawa al-Hindiyya 1/126)
Imam Ibn Abidin, the renowned classical Hanafi jurist explains in his Radd al-Muhtar that although there are narrations from some Hanafi Imams that a Sujud al-Sahw is necessary for adding a Surah in the last two rak`ahs, the chosen and more correct opinion within the School is that it is not necessary.
This is due to the fact that it is permitted to add another Surah in these rak`ahs, although better to avoid. (Radd al-Muhtar ala al-Durr al-Mukhtar 1/459, See also: Fatawa Mahmudiyyah /7412 and Ahsan al-Fatawa 4/50)
Almighty Allah knows best.