Musnad Ahmad 31:268

Abu Usamah narrated to us, he said: Fitr narrated to me, from Isma'il ibn Raja', from his father, from Abu Sa'id al-Khudri, who said: We were with the Messenger of Allah, may Allah bless him and grant him peace, and he said: "Among you is he who will fight for the interpretation of the Qur'an just as he fought for its revelation."