Musannaf Ibn Abi Shayba 3:1553

Hushaym narrated to us, he said: Husayn informed us, he said: I heard ‘Amr ibn Maymun al-Awdi saying: They would not abandon four [rak'ahs] before the Dhuhr [prayer] and two rak'ahs before the Fajr [prayer] under any circumstances.