Musannaf Ibn Abi Shayba 3:6222

Hushaym narrated to us, he said: Abu Bishr informed us, he said: A man informed [us] from Salim ibn Abd Allah, that Hafsa, the Mother of the Believers, said: "The middle prayer is the afternoon prayer (Salat al-Asr)." "And stand before Allah, devoutly obedient."