Sunan An-Nasai, Chapter: The Book of Salah

the freed slave of 'Aishah the wife of the Prophet (saw), "Aishah told me to copy a Mushaf for her, and she said: 'When you reach this verse, call my attention: Guard strictly the Salawat especially the middle (Al-Wusta) Salah. [2:238] When I reached it, I called her attention and she dictated to me: 'Guard strictly the Salawat especially the middle (Al-Wusta) Salah and the 'Asr prayer, and stand before Allah with obedience.' Then she said: 'I heard it from the Messenger of Allah (saw).'" (Sahih)