Musannaf Ibn Abi Shayba 3:5217

Muhammad ibn Fudayl narrated to us, from al-A’mash, from Muslim, who said: I used to sit with Masruq and Abu Ubaydah in the mosque during the time of Ziyad. When the time for Dhuhr entered, they would both stand and pray, then they would sit until the caller to prayer (mu'adhdhin) gave the call and the Imam came out; then they would stand and pray. And they would do the same for Asr.