Sahih Muslim, Chapter: Prayers (Kitab Al-Salat)

on the authority of Abu Huraira who said: Recitation (of Surat al-’Fatiha) in every (rak'ah) of prayer in essential. (The recitation) that we listened to from the Apostle of Allah (may peace be upon him) we made you listen to it. And that which he recited inwardly to us, we recited it inwardly for you. And he who recites Umm al-’Qur'an, it is enough for him (to complete the prayer), and he who adds to it (recites some other verses of the Holy Qur'an along with Surat al-’Fatiha), it is preferable for him.