Sayyiduna Samurah (may Allah be pleased with him) narrates concerning this saying of Allah, the Mighty and Majestic: "Guard the prayers, and especially the middle prayer" [Surah al-Baqarah: 238]. He says: the Messenger of Allah (peace and blessings be upon him) said: "It is the Asr prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6794