Musnad Abu Ya'la al-Mawsili 6:4

'Umar said to Sa'd: "They have complained about you regarding everything, even the prayer." He said: "As for me, I lengthen the first two [rak'ahs] and shorten the last two, and I do not fall short in what I have 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 "That is my thought of you."