Musnad Ahmad 5:746

Bahz narrated to us, Hammam narrated to us, from Qatadah, from Abu Hassan, from Ubaydah, who said: We used to think that the middle prayer was the morning prayer. He said: Then Ali narrated to us that on the day of the Confederates (al-Ahzab), they fought us and detained us from the afternoon prayer. The Prophet, peace and blessings of Allah be upon him, said: "May Allah fill their graves with fire—or fill their bellies with fire—just as they detained us from the middle prayer." He said: So we knew on that day that the middle prayer is the afternoon prayer.