Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 778
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: One is not credited with having observed the prayer without the recitation (of al-Fatiha). So said Abu Huraira: (The prayer in which) the Messenger of Allah (may peace be upon him) recited in a loud voice, we also recited that loudly for you (and the prayer in which) he recited inwardly we also recited inwardly for you (to give you a practical example of the prayer of the Holy Prophet).