Musnad Ahmad 1557

It was narrated that Jabir bin Samurah said: The people of Koofah complained about Sa"d to ʻUmar, and said: He does not pray properly, "Umar mentioned that to him and he said: The way the Messenger of Allah (ﷺ) prayed is how I lead them in prayer; I make it longer in the first two (rak"ahs) and shorter in the last two. He said: That is what i thought of you, O Abu Ishaq.