Jami` at-Tirmidhi 2982

Narrated Abu Yunus, the freed slave of 'Aishah:"'Aishah ordered me to write a Mushaf for her, and she said: 'When you get to this Ayah then tell me: Guard strictly (the five obligatory) prayers, and the middle Salat (2:238). So when I reached it, I told her and she dictated to me: 'Guard strictly (the five obligatory) prayers, and the middle Salat, and Salat Al-'Asr. And stand before Allah with obedience.' She said: 'I heard that from the Messenger of Allah (ﷺ)