Musannaf Ibn Abi Shayba 3:5386

Zayd ibn al-Hubab narrated to us, from al-Dahhak ibn Uthman, who said: I saw Umar ibn Abd al-Aziz praying the Dhuhr prayer with us. He would prolong the first two rak'ahs and shorten the last two. And he would shorten [the prayer] in the Asr.