Sahih Bukhari
Sahih Bukhari Volume 001, Book 010, Hadith Number 523.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 10. Times of the Prayers
Narated By Anas bin Malik : We used to pray the Asr prayer and after that if someone happened to go to the tribe of Bani Amr bin Auf, he would find them still praying the Asr (prayer).