It is narrated from Hazrat Ubadah bin Samit (may Allah be pleased with him) that once the Holy Prophet (peace and blessings be upon him) led us in prayer. During the recitation, he (peace be upon him) felt a heaviness upon himself. After finishing the prayer, he asked us, "Do you also recite (behind the Imam)?" We replied, "Yes, O Messenger of Allah!" The Holy Prophet (peace and blessings be upon him) said, "If you do not do so, there is no sin upon you, except that you must recite Surah Al-Fatihah, for without it, the prayer is not valid."