From Sahih Bukhari (Hadith 579):
Narrated Abu Huraira:Allah's Messenger (ﷺ) 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
Reference: Bukhari 579