Musnad Ahmad 27:678

Muhammad ibn Ja'far narrated to us, Shu'bah and Hajjaj narrated to us, he said: Shu'bah narrated to me, from Qatadah, from al-Mughirah ibn Salman—Hajjaj said in his narration: I heard al-Mughirah ibn Salman say: I heard Ibn Umar say: "The prayer of the Messenger of Allah, peace and blessings of Allah be upon him, which he would not abandon, was two rak'ahs before Dhuhr and two rak'ahs after it, two rak'ahs after Maghrib, two rak'ahs after Isha, and two rak'ahs before Subh."