From Sahih Muslim (Hadith 1427):
Abu Yunus, the freed slave of 'A'isha said:'A'isha 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. 'A'isha said: This is how I have heard from the Messenger of Allah (ﷺ)
Reference: Muslim 1427