كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5
The Book of Mosques and Places of Prayer
(36)
Chapter: The evidence for those who say that “the middle prayer” is the `Asr prayer
(36)
باب الدَّلِيلِ لِمَنْ قَالَ الصَّلاَةُ الْوُسْطَى هِيَ صَلاَةُ الْعَصْرِ ‏

Abu Yunus, the freed slave of 'A'isha said:

'A'isha ordered me to transcribe a copy of the Qur'an for her and said: When you reach this verse:" Guard the prayers and the middle prayer" (ii. 238), inform me; so when I reached it, I informed her and she gave me dictation (like this): Guard the prayers and the middle prayer and the afternoon prayer, and stand up truly obedient to Allah. 'A'isha said: This is how I have heard from the Messenger of Allah (ﷺ).
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى التَّمِيمِيُّ، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنِ الْقَعْقَاعِ بْنِ حَكِيمٍ، عَنْ أَبِي يُونُسَ، مَوْلَى عَائِشَةَ أَنَّهُ قَالَ أَمَرَتْنِي عَائِشَةُ أَنْ أَكْتُبَ لَهَا مُصْحَفًا وَقَالَتْ إِذَا بَلَغْتَ هَذِهِ الآيَةَ فَآذِنِّي ‏{‏ حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلاَةِ الْوُسْطَى‏}‏ فَلَمَّا بَلَغْتُهَا آذَنْتُهَا فَأَمْلَتْ عَلَىَّ حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلاَةِ الْوُسْطَى وَصَلاَةِ الْعَصْرِ ‏.‏ وَقُومُوا لِلَّهِ قَانِتِينَ ‏.‏ قَالَتْ عَائِشَةُ سَمِعْتُهَا مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم ‏.‏
Reference : Sahih Muslim 629
In-book reference : Book 5, Hadith 263
USC-MSA web (English) reference : Book 4, Hadith 1316
  (deprecated numbering scheme)