If anyone says a rak'ah of the 'Asr prayer before sunset, he has observed (the 'Asr prayer), and if anyone performs a rak'ah of the Fajr prayer, he has observed (the Fajr prayer).
Sunan Abi Dawud Book 2, Hadith 22
← Previous Hadith The Messenger of Allah (ﷺ) used to offer the Zuhr prayer in midday heat; and no prayer was harder on the Companions of the Messenger of Allah (ﷺ) that this one. Hence the revelation came down: "Be guardians of your prayers, and of the midmost prayer" (2:238). He (the narrator) said: There are two prayers before it and two prayers after it.

Sunan Abi Dawud Book 2, Hadith 21 Next Hadith → We came upon Anas b. Malik after the Zuhr prayer. He stood for saying the 'Asr prayer. When he became free from praying, we mentioned to him about observing prayer in its early period or he himself mentioned it. He said: I heard the Messenger of Allah (ﷺ) say: This is how hypocrites pray, this is how hypocrites pray, this is how hypocrites pray: He sits (watching the sun), and when it becomes yellow and is between the horns of the devil, or is on the horns of the devil, he rises and prays for rak'ahs quickly, remembering Allah only seldom during them.

Sunan Abi Dawud Book 2, Hadith 23