Musnad Ahmad 8:109

Sufyan narrated to us, from Abd al-Malik, who heard it from Jabir ibn Samurah: The people of Kufa complained about Sa’d to Umar, and they said: "He does not perform the prayer well." Sa’d said: "By Allah, I do not fall short in performing the prayer of the Messenger of Allah (peace and blessings of Allah be upon him) for them. In the Dhuhr and Asr, I prolong the first two [rak'ahs] and shorten the last two." I heard Umar say: "That is what I thought of you, O Abu Ishaq."