Musnad Abu Ya'la al-Mawsili 126:4

Muhammad ibn Bakkar, the freed slave of Banu Hashim, narrated to us, [saying]: 'Ataf ibn Khalid al-Makhzumi narrated to us, [saying]: Zayd ibn Aslam narrated to us, [saying]: I prayed the Dhuhr [prayer] with Umar ibn Abd al-Aziz, then we departed to Anas ibn Malik, may Allah be pleased with him. When we entered upon him, he said: "Have you prayed?" We said: "Yes." He said: "O slave girl, bring me water for ablution. I have not prayed behind an Imam after the Messenger of Allah, may Allah bless him and grant him peace, whose prayer was more similar to the prayer of the Messenger of Allah, may Allah bless him and grant him peace, than this Imam of yours." Zayd said: And Umar ibn Abd al-Aziz used to perfect the bowing and the prostration, and he would lighten the standing and the sitting.