Sunan An-Nasa'i

It was narrated that Abu Bakr bin 'Uthman bin Sahl bin Hunaif said:""I heard Abu Umamah bin Sahl say: 'We prayed Zuhr with 'Umar bin 'Abdul-'Aziz, then we went out and entered upon Anas bin Malik, and we found him praying 'Asr.'"" I said: ""O uncle, what is this prayer that you prayed?"" He said: ""'Asr; this is the prayer of the Messenger of Allah (ﷺ) that we used to pray with him