“Clara:
I will speak softly, till our gathered strength
Finds in its union voice. Ah, no delay!
The tyranny that dared to fetter wears
A midnight dagger.”

Source: I Shall Wear Midnight

The Vow of the Peacock (1835)

The Wearing of the Green, in Arragh na Pogue, or the Wicklow Wedding (1864)

The last line of each stanza
This is often attributed to T. S. Eliot, who does indeed quote it in The Waste Land
Prothalamion (1596)

The Observer, October 10, 1982

Song. Softly, O Midnight Hours; reported in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 721.

1900s, The Strenuous Life: Essays and Addresses (1900), National Duties
Context: Right here let me make as vigorous a plea as I know how in favor of saying nothing that we do not mean, and of acting without hesitation up to whatever we say. A good many of you are probably acquainted with the old proverb: "Speak softly and carry a big stick—you will go far." If a man continually blusters, if he lacks civility, a big stick will not save him from trouble; and neither will speaking softly avail, if back of the softness there does not lie strength, power.

Source: Battling Unbelief: Defeating Sin with Superior Pleasure