"Take due care of all the prayers, and the middle prayer, and stand before Allah in total devotion. But if you are in fear, then (pray) on foot or riding. And when you are in peace, recite the name of Allah as He has taught you what you did not know."
Based on the authority of some ahadith, a very large number of 'ulama', have said that al-salat al-wusta or the middle salah is the salah of 'Asr because there are before it, two Salahs during the day, that of Fajr and Zuhr; and after these come two evening Salahs, that of Maghrib and 'Isha'. Special emphasis has been placed on it because this is a time in which most of the people are busy in their professional work.
Incidentally, the Qur'anic word, Qanitin meaning obedient or submitting, rendered here as 'stand before Allah in total devotion', has been explained in Hadith as denoting sukut or motion-less silence. It was through this very verse that talking in salah was forbidden. Earlier, talking was permissible. The verse 239 has allowed a special way of offering prayers in the state of 'fear' i.e. the state of war. In such a state, one can offer salah (prayer) while standing, with a condition that he can stand in one place without moving, and can make the gesture of sajdah in a lower position than he makes in ruku'. However, salah cannot be performed while walking. If it is not possible to perform the prayer in the said manner, such as at the time of actualfighting, then, it is permissible to delay the prayer and to offer it later as qada'.