Sahih Bukhari
Times of the Prayers
Volume: 001 / Book Number: 010 / Hadith Number: 531
Narrated 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."