The prayer referred to as "Salat al-Wusta" (the Middle Prayer) is the Asr prayer, according to the majority of Islamic scholars and the most widely accepted interpretation. The Qur'an commands special attention to this prayer in Surah Al-Baqarah (2:238): "Guard strictly your prayers, especially the Middle Prayer (Salat al-Wusta), and stand before Allah in devout obedience."
Several authentic hadiths identify the Asr prayer as Salat al-Wusta. In one narration recorded by Muslim, the Prophet (peace be upon him) said on the day of the Battle of the Trench (Al-Khandaq): "They have kept us from the Middle Prayer, the Asr prayer." This is considered the strongest textual evidence for the identification.
The term "al-Wusta" means "the middle" and Asr falls in the middle of the five daily prayers chronologically: Fajr and Dhuhr come before it, while Maghrib and Isha come after it. The Qur'an singles it out because it occurs during a busy time of the day when people are occupied with work and trade, making it the prayer most likely to be neglected or delayed.