CAESAR: Cowards die many times before their deaths;
The valiant never taste of death but once.
Of all the wonders that I yet have heard,
It seems to me most strange that men should fear,
Seeing that death, a necessary end,
Will come when it will come.

William Shakespeare (1564-1616) English dramatist and poet

Julius Caesar, Act 2, sc. 2, l. 34ff (2.2.34-39) (1599)
    (Source)

The initial phrase has seemingly morphed in the retelling, though still being cited to Shakespeare: "A coward dies a thousand times before his death, but the valiant taste of death but once." This is the form most often seen, but is not Shakespeare.

In A Farewell to Arms (1929), Hemingway gives another paraphrase: "The coward dies a thousand deaths, the brave but one," obliquely attributed to Shakespeare, This, too, sometimes gets modified to make it scan better, e.g., "A coward dies a thousand deaths, the brave dies only once." He goes on to write, "The brave dies perhaps two thousand deaths if he’s intelligent. He simply doesn’t mention them."