Ubaidah (may Allah have mercy on him) says that we used to consider the middle prayer (Salat al-Wusta) to be the Fajr prayer, then one day Sayyiduna Ali (may Allah be pleased with him) narrated this hadith that during the Battle of Ahzab, when the fighting began, the polytheists prevented us from performing the Asr prayer. On that occasion, the Prophet (peace and blessings be upon him) said: "O Allah! Fill their bellies and graves with fire, for they did not allow us to perform the Asr prayer until the sun had set." On that day, we realized that the middle prayer (Salat al-Wusta) refers to the Asr prayer.