Musnad Ahmad 27:983

Bahz narrated to us, Shu'bah narrated to us, from Qatadah, who said: I heard al-Mughirah ibn Salman narrating from Ibn Umar, who said: "Ten rak'ahs the Prophet, may Allah bless him and grant him peace, used to persist in: two rak'ahs before Dhuhr, two rak'ahs after Dhuhr, two rak'ahs after Maghrib, two rak'ahs after Isha, and two rak'ahs before Fajr."