Musnad Ahmad 8:118

Jarir ibn Abd al-Hamid narrated to 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 pray well." Umar mentioned that to him, so he said: "As for the prayer of the Messenger of Allah, may Allah bless him and grant him peace, I used to pray it with them; I would prolong the first two [rak'ahs] and shorten the last two." He [Umar] said: "That is what I thought of you, O Abu Ishaq."