It is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to lead the (Zuhr) prayer in the intense midday heat, or he said: at the time of the hajirah, and it was the heaviest prayer on the Companions. So this verse was revealed: "Guard strictly the prayers, especially the middle prayer, and stand before Allah in humble devotion" [Surah al-Baqarah: 238]. Zayd (may Allah be pleased with him) said: "Because there are two prayers before it and two prayers after it."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4804