Sunan Abi Dawud

Zaid b. Thabit said: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