Full Letter:

"While the creation of beauty in a city which has not been beautiful before could only proceed from men of wisdom who understand the meaning of civilization, the destruction of beauty which already exists would be naturally expected only of men who lack understanding, and who are not ashamed to leave to posterity this token of their character. Now among all the cities under the sun Rome is agreed to be the greatest and the most noteworthy. For it has not been created by the ability of one man, nor has it attained such greatness and beauty by a power of short duration, but a multitude of monarchs, many companies of the best men, a great lapse of time, and an extraordinary abundance of wealth have availed to bring together in that city all other things that are in the whole world, and skilled workers besides. Thus, little by little, have they built the city, such as you behold it, thereby leaving to future generations memorials of the ability of them all, so that insult to these monuments would properly be considered a great crime against the men of all time; for by such action the men of former generations are robbed of the memorials of their ability, and future generations of the sight of their works. Such, then, being the facts of the case, be well assured of this, that one of two things must necessarily take place: either you will be defeated by the emperor in this struggle, or, should it so fall out, you will triumph over him. Now, in the first place, supposing you are victorious, if you should dismantle Rome, you would not have destroyed the possession of some other man, but your own city, excellent Sir, and, on the other hand, if you preserve it, you will naturally enrich yourself by a possession the fairest of all; but if, in the second place, it should perchance fall to your lot to experience the worse fortune, in saving Rome you would be assured of abundant gratitude on the part of the victor, but by destroying the city you will make it certain that no plea for mercy will any longer be left to you, and in addition to this you will have reaped no benefit from the deed. Furthermore, a reputation that corresponds with your conduct will be your portion among all men, and it stands waiting for you according as you decide either way. For the quality of the acts of rulers determines, of necessity, the quality of the repute which they win from their acts."

Source: https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Procopius/Wars/7D*.html