Muwatta Malik, Book 8, Hadith 27 (USC-MSA)

Yahya related to me from Malik from Zayd ibn Aslam that Amr ibn Rafi said, "I was writing a Qur'an for Hafsa, umm al-muminin, and she said, 'When you reach this ayat, let me know, "Guard the prayers carefully and the middle prayer and stand obedient to Allah." When I reached it I told her and she dictated to me, 'Guard the prayers carefully and the middle prayer and the asr prayer and stand obedient to Allah.' "

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ عَمْرِو بْنِ رَافِعٍ، أَنَّهُ قَالَ كُنْتُ أَكْتُبُ مُصْحَفًا لِحَفْصَةَ أُمِّ الْمُؤْمِنِينَ فَقَالَتْ إِذَا بَلَغْتَ هَذِهِ الآيَةَ فَآذِنِّي ‏{‏حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلاَةِ الْوُسْطَى وَقُومُوا لِلَّهِ قَانِتِينَ ‏}‏ فَلَمَّا بَلَغْتُهَا آذَنْتُهَا فَأَمْلَتْ عَلَىَّ حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلاَةِ الْوُسْطَى وَصَلاَةِ الْعَصْرِ وَقُومُوا لِلَّهِ قَانِتِينَ ‏.‏

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also referenced under: Book 8, Hadith 316 (In-Book)