Sahih Bukhari
Sahih Bukhari Volume 001, Book 010, Hadith Number 531.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 10. Times of the Prayers
Narated By Abu Huraira : Allah's Apostle said, "If anyone of you can get one Rak'a of the 'Asr prayer before sunset, he should complete his prayer. If any of you can get one Rak'a of the Fajr prayer before sunrise, he should complete his prayer."