We used to think that the middle prayer was Fajr prayer. Then ʿAli ؓ told us that on the day of al-Ahzab they fought and were distracted from ‘Asr prayer. The Prophet ﷺ said: ʿO Allah, fill their graves with fire - or fill their bellies with fire - as they kept us from the middle prayer.ʿ Then on that day we realised that the middle prayer was ’Asr prayer.
فَحَدَّثَنَا عَلِيٌّ أَنَّهُمْ يَوْمَ الْأَحْزَابِ اقْتَتَلُوا وَحَبَسُونَا عَنْ صَلاةِ الْعَصْرِ فَقَالَ النَّبِيُّ ﷺ اللهُمَّ امْلأ قُبُورَهُمْ نَارًا أَوِ امْلأ بُطُونَهُمْ نَارًا كَمَا حَبَسُونَا عَنْ صَلاةِ الْوُسْطَى قَالَ فَعَرَفْنَا يَوْمَئِذٍ أَنَّ صَلاةَ الْوُسْطَى صَلاةُ الْعَصْرِ