Musnad Abu Ya'la al-Mawsili 6:55

Abu Khaythama narrated to us, Ibn ‘Uyaynah narrated to us, from ‘Abd al-Malik ibn ‘Umayr, from Jabir ibn Samurah, who said: I heard ‘Umar saying to Sa‘d: "The people of Kufa have complained about you, to the point that they said: 'He does not perform the prayer well.'" He said: "Me?" He said: "Yes." He said: "By Allah, I would not fall short for them regarding the prayer of the Messenger of Allah (may Allah bless him and grant him peace). I would prolong the first two [rak'ahs] and shorten the last two." So I heard ‘Umar saying to him: "That is the expectation of you, that is the expectation of you."