“While there's life there’s hope, and only the dead have none.”

From a speech entitled Come September http://ada.evergreen.edu/~arunc/texts/politics/comeSeptember.pdf.
Speeches
Source: The Cost of Living

Heauton Timorumenos (The Self-Tormentor)

Source: Heauton Timorumenos (The Self-Tormentor), Line 981.

Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

Captain Roadstrum to Bjorn, on planet Valhal, also known as Lamos, in Ch. 2
Space Chantey (1968)
Context: I do not understand your custom in this, but we do not intend to fight until all of us are dead. We desire very much that none of us be dead. And we will fight till all of you are dead only if it is absolutely necessary.

Fable, The Sick Man and the Angel
Comparable to: "For the living there is hope, but for the dead there is none", Theocritus (3rd century BC), Idyl iv, 42; "Ægroto, dum anima est, spes est" ("While the sick man has life, there is hope", Cicero (1st century BC), Epistolarum ad Atticum, ix, 10
Fables (1727)

Attributed to Plato by General Douglas MacArthur, earliest source found is work of George Santayana who doesn't attribute it to anyone. Plato and his dialogues by Bernard SUZANNE, "Frequently Asked Questions about Plato : Did Plato write "Only the dead have seen the end of war"?" http://plato-dialogues.org/faq/faq008.htm
Source: Soliloquies in England and Later Soliloquies (1922), "Tipperary"

Pero la vida es corta:
viviendo, todo falta;
muriendo, todo sobra.
Act III, sc. vii. Translation from Arthur Terry Seventeenth-Century Spanish Poetry (Cambridge: CUP, 1993) p. 118.
La Dorotea (1632)

At A Child's Grave (1882)
Context: The dead do not suffer. And if they live again, their lives will surely be as good as ours. We have no fear. We are all children of the same mother, and the same fate awaits us all.
We, too, have our religion, and it is this: Help for the living, hope for the dead.