Musnad Ahmad 8:72

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu 'Awn, from Jabir ibn Samurah. And Bahz and 'Affan said: Shu'bah narrated to us, Abu 'Awn informed me, Bahz said: He said: I heard Jabir ibn Samurah say: Umar said to Sa'd: "The people complained about you in everything, even in the prayer." He said: "As for me, I prolong the first two [rak'ahs], and I shorten the last two, and I do not fall short in what I followed of the prayer of the Messenger of Allah, peace and blessings of Allah be upon him." Umar said: "That is the thought of you—or my thought of you."