Sahih Bukhari Volume 1, Book 10, Hadith Number 553

Narated By Abu Huraira : Allah’s Apostle said, “Whoever could get one Rak’a (of the Fajr prayer) before sunrise, he has got the (morning) prayer and whoever could get one Rak’a of the’Asr prayer before sunset, he has got the (‘Asr) prayer.”