Sahih Bukhari
Times of the Prayers
Volume: 001 / Book Number: 010 / Hadith Number: 553
Narrated 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."