“In painting as in eloquence, the greater your strength, the quieter your manner.”

Source: A Mother's Advice to Her Son, 1726, p. 165

Speaker's Handbook of Epigrams and Witticisms (1955), p. 187.
Attributed

Speech in West Calder, Scotland (27 November 1879), quoted in W. E. Gladstone, Midlothian Speeches 1879 (Leicester University Press, 1971), p. 116.
1870s
Context: My fourth principle is—that you should avoid needless and entangling engagements. You may boast about them, you may brag about them, you may say you are procuring consideration of the country. You may say that an Englishman may now hold up his head among the nations. But what does all this come to, gentlemen? It comes to this, that you are increasing your engagements without increasing your strength; and if you increase your engagements without increasing strength, you diminish strength, you abolish strength; you really reduce the empire and do not increase it. You render it less capable of performing its duties; you render it an inheritance less precious to hand on to future generations.

Music, Slippery When Wet (1986)

Source: Sex, Drugs, and Cocoa Puffs: A Low Culture Manifesto