I offered the obligatory prayer and did not remember the Surah (i.e. chapter of the Noble Quran) which I recited. What is the ruling for this?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If you offered the obligatory prayer, recited Al-Faatihah (i.e. the opening chapter of the Noble Quran) in every Rak‘ah and recited some other verses which you know from the Noble Quran along with it in the Rak‘ahs in which it is a Sunnah to do, then your prayer is valid and in conformity with the Sunnah. There is no harm in not remembering the Surah that you recited. Also, this has no effect on the validity of the prayer, although submission and attention of the heart are required when offering prayer. If you observe this while doing so, you may not forget what you recited along with Al-Faatihah. However, it should be known that reciting Al-Faatihah is a must in every Rak‘ah for it is one of the pillars of prayer.

‘Ubaadah ibn As-Saamit narrated that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “There is no prayer for one who does not recite Umm Al-Kitaab (i.e. Surat al-Faatihah).” [Muslim, Abu Daawood, and An-Nasaa’i]

Allaah Knows best.