Musnad Ahmad 1153:435

Ishaq narrated to us, he said: Malik informed me, from Zayd ibn Aslam, from al-Qa'qa' ibn Hakim, from Abu Yunus, the freed slave of 'Aishah, who said: 'Aishah ordered me to write a Mushaf for her. She said: "When you reach this verse: {Guard strictly the prayers, especially the middle prayer} [Al-Baqarah: 238], inform me." When I reached it, I informed her. She dictated to me: "Guard strictly the prayers, and the middle prayer, and the afternoon (Asr) prayer, and stand before Allah in obedience." She said: "I heard it from the Messenger of Allah, may Allah bless him and grant him peace."