Sahih Muslim Book 4, Hadith Number 1316.

Chapter : Preference for saying the ‘Asr prayer at the commencement of the prescribed time.

Abu Yunus, the freed slave of ‘Aisha said: ‘Aisha ordered me to transcribe a copy of the Qur’an for her and said: When you reach this verse: “Guard the prayers and the middle prayer” (ii. 238), inform me; so when I reached it, I informed her and she gave me dictation (like this): Guard the prayers and the middle prayer and the afternoon prayer, and stand up truly obedient to Allah. ‘Aisha said: This is how I have heard from the Messenger of Allah (may peace be upon him).