Musnad Ahmad 32:1522

Muhammad ibn Abdullah al-Ansari narrated to us, Humayd al-Tawil narrated to us, from Anas ibn Malik, who said: "The prayer of the Messenger of Allah, peace and blessings of Allah be upon him, was moderate [in length]. The prayer of Abu Bakr was intermediate. And Umar would lengthen the recitation in the morning prayer."