Musnad Ahmad 1518

It was narrated that Jabir said: The people of Koofah complained to "Umar (رضي الله عنه) about Sa"d (رضي الله عنه) and said: “He does not pray properly.” ‘Umar (رضي الله عنه) asked him and he said: “I lead them in prayer as the Messenger of Allah (ﷺ) prayed; I make it long in the first two (rak"ahs) and I make it brief in the last two.” He said: “That is what I thought you would do, O Abu Ishaq.”