Musnad Abu Ya'la al-Mawsili 129:100

Abu Bakr ibn Abi Shaybah narrated to us, al-Thaqafi narrated to us, from Humayd, from Anas, who said: The prayer of the Messenger of Allah (may Allah bless him and grant him peace) was close to one another [in length], and the prayer of Abu Bakr (may Allah be pleased with him) was close [in length]. Then Umar extended the morning prayer.