Musnad Ahmad 31:921

'Affan narrated to us, Hammam narrated to us, Qatadah informed us, from Abu Nadrah, from Abu Sa'id al-Khudri, who said: "Our Prophet, may Allah bless him and grant him peace, commanded us to recite the Opening of the Book (Al-Fatihah) and whatever is made easy (of the Quran)."