Question:

A person was perfoming four rakaats of fardh salah and he forgot to recite the surah after fatihah in the second rakat. He directly went into ruku. Then after going into ruku he remembered that he did not recite the surah. So in such a scenario, if he does not return to recite the surah after remembering about it in ruku, will his salah become invalid?

Answer: حامدا ومصليا ومسلما

In this scenario when he remembered in ruku that he did not recite the surah, it is preferable for him to return to the standing position, recite the surah, then repeat the ruku and finally perform sajdah sawh.

However if he did not return to the standing position, rather he continued his salah, it will not become null and void. However, sajdah sahw will be wajib.

ولو تذكرها في ركوعه قرأَها و أَعاد الركوع (قوله : ولو تذكرها) اي السورة (قوله : قرأَها) اي بعد عوده الى القيام الخ (الدر المختار مع الشامي 1:547)

From the above, this is also learnt, that if this person returned to the standing position to recite the surah, then in such a case it is necessary to repeat the ruku after reciting the surah. If he does not repeat the ruku his salah will become invalid. You probably misunderstood from this, ‘that if the person did not return to the standing position to recite the surah his salah will become invalid.’ But that is not the case.

And Allah Ta’aalaa knows best.
Hazrat Mufti Ahmad Khanpuri Saheb (May Allah preserve him)
Mahmoodul Fataawaa 1/458-459