Be true to your work, your word, and your friend.

Henry David Thoreau (1817-1862) American philosopher and writer

(Misattributed)

Actually from James Jeffrey Roche, "Rules of the Road" (1891):

Be silent and safe — silence never betrays you;
Be true to your word and your work and your friend;
Put least trust in him who is foremost to praise you,
Nor judge of a road till it draw to the end.