Musnad Ahmad 5:426

Abdullah narrated to us, Abu Ishaq al-Tirmidhi narrated to me, al-Ashja‘i narrated to us, from Sufyan, from ‘Asim, from Zirr ibn Hubaysh, from ‘Ubaydah al-Salmani, from ‘Ali, who said: We used to consider it the dawn prayer (Fajr). Then the Messenger of Allah (peace and blessings of Allah be upon him) said: "It is the afternoon prayer (Asr)," meaning the middle prayer.