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
← Previous Hadith 'Aishah commanded me to write for her come passage from the Qur'an. She also added: When you reach the following verse, inform me: "Be guardian of your prayers and of the midmost prayer" (2:238). When I reached it, I informed her. She asked me to write: "Be guardians of your prayers, and of the midmost prayer, and of the 'Asr prayer, and stand up with devotion of Allah" (2:238). 'Aishah then said: I heard it from the Messenger of Allah (ﷺ).

Sunan Abi Dawud Book 2, Hadith 20 Next Hadith → 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