To sin by silence, when we should protest,
Makes cowards out of men.

Ella Wheeler Wilcox (1850-1919) American author, poet, temperance advocate, spiritualist

Poem (1914), “Protest,” ll. 1-2, Poems of Problems
    (Source)

Often misattributed to Abraham Lincoln, after Douglas MacArthur did so in a 1950 speech.

See Confucius.