The Messenger of Allah ﷺ used to pray Zuhr at the height of the midday heat, and no prayer was harder on the companions of the Messenger of Allah ﷺ than this one. So the verse was revealed, "Guard strictly the prayers, and the middle prayer" [Baqarah 2:238]. He said, "Indeed, before it are two prayers and after it are two prayers."
حدثنا محمد بن المثنى حدثني محمد بن جعفر حدثنا شعبة حدثني عمرو بن أبي حكيم قال سمعت الزبرقان يحدث عن عروة بن الزبير
كان رسول الله ﷺ يصلي الظهر بالهاجرة ولم يكن يصلي صلاة أشد على أصحاب رسول الله ﷺ منها فنزلت {حافظوا على الصلوات والصلاة الوسطى} وقال إن قبلها صلاتين وبعدها صلاتين
Sourcing, its chain of transmission is sahih. Nasai narrated it in his Kubra, no. 357, from Muhammad bin Muthanna, and Ahmad, 5/183, from Muhammad bin Jafar, with the same wording. Ibn Hazm graded it sahih in Muhalla, 4/250, and said, "There is no clear indication here that it refers to Dhuhr."