Musnad Abu Ya'la al-Mawsili 3:130

Ubayd Allah ibn Umar narrated to us, Yahya ibn Sa'id narrated to us, from Sufyan, from Asim, from Zirr, who said: Ubaydah ordered us to ask Ali about the middle prayer. So I asked him, and he said: "I used to think that it was the dawn prayer, until I heard the Messenger of Allah (may Allah bless him and grant him peace) say on the day of the Confederates (al-Ahzab): 'They distracted us from the middle prayer, the afternoon prayer (Asr). May Allah fill their graves and their bellies with fire.'"