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: Sahih al-Bukhari 579 In-book : Book 9, Hadith 55 USC-MSA web (English) : Vol. 1, Book 10, Hadith 553 (deprecated)
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