Musnad Abu Ya'la al-Mawsili 6:53

'Ali ibn 'Abdullah ibn Ja'far ibn Najih al-Madini narrated to us, Yahya ibn Sa'id narrated to us, Shu'bah narrated to us, Abu 'Awn narrated to us, he said: I heard Jabir ibn Samurah saying: 'Umar said to Sa'd: "The people of Kufa have complained about you regarding everything, even regarding the prayer." He said: "I lengthen the first two [rak'ahs] and I shorten the last two, and I do not fall short of what I followed of the prayer of the Messenger of Allah, may Allah bless him and grant him peace." He said: "That is the thought of you," or "my thought of you."