Ruling on reciting a surah with al-Fatihah in the third and fourth rak'ahs

Question:
I hear, while I am praying, someone reciting surahs from the Qur'an in the third and fourth rak'ahs. Is this part of the Sunnah? And does it affect the validity of the prayer?

Answer:
It is established in the two Sahihs from Abu Qatadah al-Ansari (may Allah be pleased with him) from the Prophet (peace and blessings be upon him) that he used to recite in the third and fourth rak'ahs only al-Fatihah in the Dhuhr and 'Asr prayers, and likewise in the 'Isha' and Maghrib prayers. So the Sunnah is to restrict oneself to al-Fatihah in the third and fourth rak'ahs of the Dhuhr, 'Asr, Maghrib, and 'Isha' prayers.
However, it is recommended for him at times—especially in the Dhuhr prayer—to recite more; because it is established in the hadith of Abu Sa'id that indicates that he (peace and blessings be upon him) sometimes used to recite more in the third and fourth rak'ahs of the Dhuhr prayer specifically. So if he recites more in the Dhuhr and 'Asr in the third and fourth rak'ahs, somewhat along with al-Fatihah, there is no harm.
And it is recommended at times, not always; in order to reconcile the hadiths reported on this matter, because Abu Qatadah mentioned that he used to recite only al-Fatihah in the third and fourth rak'ahs, and Abu Sa'id mentioned what indicates that at times he used to recite more than al-Fatihah, yes, in the Dhuhr prayer specifically.