Musnad Ahmad 27:945

Hassan narrated to us, Ibn Lahi'ah narrated to us, Ja'far ibn Rabi'ah narrated to us, from Abd al-Rahman ibn Rafi' al-Hadrami, who said: I saw Ibn Umar at the prayer ground on the day of Fitr, and beside him was a son of his. He said to his son: "Do you know how the Messenger of Allah, may Allah bless him and grant him peace, used to do on this day?" He said: "I do not know." Ibn Umar said: "The Messenger of Allah, may Allah bless him and grant him peace, used to pray before the sermon."