Musnad Ahmad 1548

It was narrated from ‘Abdul-Malik, who heard it from Jabir bin Samurah: The people of Koofah complained to ‘Umar about Sa"d and said: He does not pray properly. He said: The Bedouin? By Allah, I strive hard to make my prayer with them as much like the prayer of the Messenger of Allah (ﷺ) as I can. In Zuhr and ‘Asr I make the first two rak"ahs long and I make the last two short, I heard ‘Umar say: That is what I thought of you, O Abu Ishaq.