Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.085.

Says Allah in the Qur’an, “Observe and guard the prayers and the mid-most prayer, and stand with total submission to Allah. Authentic hadith have made it clear that the afternoon prayers is the “mid-most” prayer.

‘Ali reported that the Prophet, upon whom be peace, said on the day of al-Ahzab (the battle of the clans), “May Allah fill their graves and houses with fire, as they kept us preoccupied from the “mid-most” prayer until the sun had set.” (Related by al-Bukhari and Muslim. Muslim, Abu Dawud and Ahmad have “the afternoon prayer” inserted after “the mid-most” prayer.”)

Explaining the context of this hadith, Ibn Mas’ud said, “The idol-worshippers kept the Prophet from the afternoon prayer until the sun had become reddish and yellowish. The Messenger of Allah said, “They kept us preoccupied from the “mid-most” prayer, the afternoon prayer–may Allah fill their bellies and graves with fire.”‘ (Related by Ahmad, Muslim, and Ibn Majah.)