Sayyiduna Samurah (may Allah be pleased with him) says: The Messenger of Allah (peace and blessings be upon him) ordered us to guard all the prayers, and the Messenger of Allah (peace and blessings be upon him) gave special instruction regarding the middle prayer, and told us that it is the Asr prayer.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6979