Musnad Ahmad 31:425

Bahz and ‘Affan narrated to us, they said: Hammam narrated to us, from Qatadah—‘Affan said: Qatadah narrated to 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)."