From Sahih Bukhari (Hadith 556):
Narrated Abu Huraira:Allah's Messenger (ﷺ) 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
Reference: Bukhari 556