It was narrated from 'Aishah that the Prophet (ﷺ) said:
"Whoever catches up with a Rak'ah of Fajr before the sun rises, then he has caught up with it; and whoever catches up with a Rak'ah of 'Asr before the sun sets, then he has caught up with it." (FR)
English reference (FR): Vol. 1, Book 6, Hadith 552 (FR)
Arabic reference (FR): Book 6, Hadith 556 (FR)