Sahih Bukhari Volume 001, Book 010, Hadith Number 524.
Narated By Abu Bakr bin Uthman bin Sahl bin Hunaif : That he heard Abu Umama saying: We prayed the Zuhr prayer with 'Umar bin Abdul Aziz and then went to Anas bin Malik and found him offering the Asr prayer. I asked him, "O uncle! Which prayer have you offered?" He said 'The Asr and this is (the time of) the prayer of Allah s Apostle which we used to pray with him."