Tart Words make no Friends: a spoonful of honey will catch more flies than Gallon of Vinegar.

Benjamin Franklin (1706–1790) American statesman, scientist, philosopher, aphorist
Poor Richard (1744 ed.)
    (Source)

Not original with Franklin. See Giovanni Torriano, A Common Place of Italian Proverbs [Piazza universale di proverbi Italiani] (1666):

Il mele catta più mosche, che non fà l'aceto.
 
Honey gets more flyes to it, than doth viniger.