Musannaf Ibn Abi Shayba 3:5380

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: 'Abd al-Malik ibn 'Umayr informed us, he said: Jabir ibn Samurah narrated to us that some people complained about Sa'd to 'Umar ibn al-Khattab. He said: And they complained about him regarding the prayer. So 'Umar wrote to him, and he came to him. He said: So he mentioned that which they complained about him regarding, and he mentioned that they complained about him regarding the prayer. Sa'd said: "I indeed pray with them the prayer of the Messenger of Allah (may Allah bless him and grant him peace). I indeed prolong for them in the first two [rak'ahs], and I shorten for them in the last two." He said: "That is the expectation of you, O Abu Ishaq."