Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

that once while Allah's Mesenge (SAW) led the fajr salah, the recitation of the Qur'an became difficult for him. When he finished, he said, "Perhaps you recite behind your imam." They replied, "By Allah, yes, 0 Messenger of Allah!" He said, "Do not do it. Recite only the umm ul-Qur'an (surah al-Fatihah), for the one who does not recite it, has not offered salah."