Sahih Muslim Book 4, Hadith Number 1317.

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. Imam Muslim said: Ashja’i narrated it from Sufyan al-Thauri, who narrated it from al-Aswad b. Qais, who narrated it from ‘Uqba, who narrated it from al-Bara’ b. ‘Azib who said: We recited with the Prophet (may peace be upon him) (the above-mentioned verse like this, i.e. instead of Salat al-Wusta, Salat al-‘Asr) for a certain period, as it has been mentioned (in the above-quoted Hadith).