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: Sahih al-Bukhari 556 In-book : Book 9, Hadith 33 USC-MSA web (English) : Vol. 1, Book 10, Hadith 531 (deprecated)
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