Mukhtasar Muslim (Abridged) 217 — What is meant by the middle prayer?

´Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said that` the polytheists prevented the Messenger of Allah (peace and blessings of Allah be upon him) from the Asr prayer, until the sun became red or yellow. Then the Prophet (peace and blessings of Allah be upon him) said: They have prevented us from the Asr prayer, the middle prayer. May Allah fill their bellies and graves with fire. (Or he said: May Allah fill their insides and graves with fire.)