Musnad Ahmad 8:80

'Abd al-Razzaq narrated to us, Sufyan informed us, from 'Abd al-Malik ibn 'Umayr, from Jabir ibn Samurah, who said: The people of Kufa complained about Sa'd to 'Umar, and they said: "He does not perform the prayer well." He said: So 'Umar asked him, and he said: "I perform the prayer of the Messenger of Allah (may Allah bless him and grant him peace) with them; I prolong the first two [rak'ahs], and I shorten the last two." He said: "That is what I thought of you, O Abu Ishaq."