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.