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 offer (the Zuhr prayer) at midday. This was very burdensome for his Companions (may Allah be pleased with them). Then this verse was revealed: "Guard the prayers, especially the middle prayer, and stand before Allah in devout obedience." [Surah al-Baqarah: 238] (Sayyiduna Zayd bin Thabit (may Allah be pleased with him)) said: This is the middle prayer, because there are two prayers before it and two prayers after it.
In another narration the wording is that "he used to offer the Zuhr prayer at midday."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2154