What Is Salat Al-Asalah?

M'kathir and Zumashiri both commented, this is Salat al-Asar. They're an interpretation of what Al-Asar was,. And this is part of the methodology of Tefsir that says that when Allah swears why something, it must be something sacred. So in line with that methodology, they are saying, because SalatAlAsar is sacred, how is it sacred?Allah calls it Hafidu al-Asalawatikum: guard your prayers, especially the middle prayer.