Politeness is prudence and consequently rudeness is folly. To make enemies by being wantonly and unnecessarily rude is as crazy as setting one’s house on fire.

[Höflichkeit ist Klugheit; folglich ist Unhöflichkeit Dummheit: sich mittelst ihrer unnötiger und mutwilliger Weise Feinde machen ist Raserei, wie wenn man sein Haus in Brand steckt.]

Arthur Schopenhauer (1788-1860) German philosopher

Parerga and Paralipomena, Vol. 1, “Aphorisms on the Wisdom of Life [Aphorismen zur Lebensweisheit],” ch. 5 “Counsels and Maxims [Paränesen und Maximen],” § 3.36 (1851) [tr. Payne (1974)]
    (Source)

Source (German). Alternate translation:

It is a wise thing to be polite; consequently, it is a stupid thing to be rude. To make enemies by unnecessary and willful incivility, is just as insane a proceeding as to set your house on fire.
[tr. Saunders (1890)]