Musannaf Ibn Abi Shayba 3:3567

Abu al-Ahwas narrated to us, from Husayn, from ‘Amr ibn Maymun, who said: The companions of the Prophet (may Allah bless him and grant him peace) would not leave the four rak'ahs before the Dhuhr prayer and the two rak'ahs before the Fajr prayer under any circumstances.