Dr. Abdul-Fattah Idrees, professor of Comparative Jurisprudence at Al-Azhar University, states the following: Reciting a Qur’anic Surah after Al-Fatihah is a Sunnah in the first two rak`ahs (Prayer units).
However, if one is unable to recite any verse(s) or Surah after reciting Al-Fatihah, then his Prayer remains valid.”
As for reciting a Surah after Al-Fatihah in the third and the fourth rak`ahs, Dr. Husam Ed-Din Ibn Musa `Afana, professor of Islamic Jurisprudence at the University of Al-Quds, Palestine, states the following:
“It is enough in the third and the fourth Rak`ah to recite only Al-Fatihah. This is the opinion followed by the majority of Muslim scholars and jurists. However, if the praying person recites a surah after the he third and the fourth rak`ahs, then there is nothing wrong with what he has done.”