It is narrated from Nafi' bin Mahmud bin Rabi' that he heard Ubadah bin al-Samit (may Allah be pleased with him) reciting Surah al-Fatihah while Abu Nu'aym (as the imam of the congregation) was reciting aloud. Nafi' said: I said: I heard you reciting something in the prayer? He said: What was that? I said: I heard you reciting Umm al-Qur'an (al-Fatihah) while Abu Nu'aym was reciting aloud. He said: Yes (that is so). The Messenger of Allah (peace and blessings be upon him) led us in a prayer with loud recitation. When he (peace and blessings be upon him) finished the prayer, he said: "When I recite aloud, does one of you perhaps recite?" We said: Yes, O Messenger of Allah. The Messenger of Allah (peace and blessings be upon him) said: "That is why I was thinking: what is the reason that reciting the Qur'an is becoming difficult for me? So when I am reciting aloud, do not recite anything of the Qur'an except Surah al-Fatihah."
Abu al-Hasan al-Daraqutni (may Allah have mercy on him) said: This chain is hasan (good), and its narrators are thiqat (trustworthy).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2920