Musnad Abu Ya'la al-Mawsili 129:127

Zuhayr narrated to us, Yazid bin Harun narrated to us, Humayd al-Tawil informed us, from Anas, who said: The prayer of the Messenger of Allah (may Allah bless him and grant him peace) and Abu Bakr was close [in length], until it was Umar, so he lengthened the morning prayer.