Don’t flatter yourselves that friendship authorizes you to say disagreeable things to your intimates. On the contrary, the nearer you come into relation with a person, the more necessary do tact and courtesy become.

Oliver Wendell Holmes, Sr. (1809-1894) American poet, essayist, scholar

Article (1858-01), “The Autocrat of the Breakfast-Table,” Atlantic Monthly
    (Source)

Collected in The Autocrat of the Breakfast-Table, ch. 3 (1858).