[كان إذا مشى مشى أصحابه أمامه وتركوا ظهره للملائكة]
It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) walked, his (peace and blessings be upon him) Companions would walk in front of him (peace and blessings be upon him) and leave his (peace and blessings be upon him) back for the angels."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4787