This passage comes from Hugo Grotius, The Law of War and Peace (1625), Book III Chapter 12 "On Moderation iin Despoiling an Enemy’s Country" (1625):
There are some things of such a nature, as to contribute, no way, to the support and prolongation of war: things which reason itself requires to be spared even during the heat and continuance of war: … Such are Porticos, Temples, statues, and all other elegant works and monuments of art… As this rule of moderation is observed towards other ornamental works of art, for the reasons before stated, there is still greater reason, why it should be obeyed in respect to things devoted to the purposes of religion.
About this Quotation:
The OLL has two editions of Grotius book on The Laws of War and Peace online. The 1901 edition was published at a time when a number of Conventions had been convened to modernise the laws of war and to help ward off an expected conflict between the Great Powers of Europe (which nevertheless took place in 1914). This edition contained an introduction by David J. Hill who was Assistant Secretary of State in the U.S., thus giving the project the stamp of approval of the American government. The second edition we have online is a 3 volume edition published in 2005 by Richard Tuck. It is now the definitive scholarly edition of Grotius' work and is part of a 40 volume series on The Enlightenment and Natural Law. Having lived through the early years of the Thirty Years War (1618-1648) which devastated so much of central and northern Europe it is not surprising that Grotius would be concerned about the effects of war on innocent civilians and how best to minimise this impact.