It's mentioned in Vegetius' De Re Militari. The book is a military manual written in the late 4th century outlining how Vegetius thinks the Roman army should reform itself to be more like the early Empire's military.

Vegetius says that the army of his day did not fortify camps, and should re-adopt the practice since it has led to several armies being taken by surprise at night. Vegetius makes the case that this is another product of the neglect of the previous emperors in maintaining the military, and the barbarization of the army.

This was also helped along by the fact that the borders had stabilized and more permanent fortifications were built on them, and the armies became much less mobile, thus leading to a decrease in the practice.