Musnad Ahmad 1153:1432

I read to Abd al-Rahman: Malik, from Zayd ibn Aslam, from al-Qa'qa' ibn Hakim, from Abu Yunus, the freed slave of Aisha, the wife of the Prophet, may God bless him and grant him peace, that he said: Aisha ordered me to write a Mushaf (copy of the Quran) for her. She said: "When you reach this verse, inform me: {Guard strictly the prayers, especially the middle prayer} [Al-Baqarah: 238]." He said: When I reached it, I informed her, and she dictated to me: "Guard strictly the prayers, and the middle prayer, and the afternoon prayer (Salat al-'Asr), and stand before God in obedience." Then she said: "I heard it from the Messenger of God, may God bless him and grant him peace."