(36) باب الدَّلِيلِ لِمَنْ قَالَ الصَّلاَةُ الْوُسْطَى هِيَ صَلاَةُ الْعَصْرِ
Al-Bara' b. 'Azib reported:
This verse was revealed (in this way):" Guard the prayers and the 'Asr prayer." We recited it (in this very way) so long as Allah desired. Allah, then, abrogated it and it was revealed:" Guard the prayers, and the middle prayer." A person who was sitting with Shaqiq (one of the narrators in the chain of transmitters) said: Now it implies the 'Asr prayer. Upon this al-Bara' said: I have already informed you how this (verse) was revealed and how Allah abrogated it, and Allah knows best.
Reference: Sahih Muslim 630
In-book reference: Book 5, Hadith 264
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1317