In addition to other works where it might have appeared, it is in Shakespeare's "King Henry IV, Pt I," where the wording is, "The better part of valor is discretion."

Falstaff in Shakespeare's "Henry IV, Part One," says it.

See the attached link for a brief explanation.

Falstaff, in Shakespeare's "Henry the Fourth", said "the better part of valor is discretion" but Shakespeare is notorious for, well, let's not call it theft. He was inspired, yeah, that's the ticjet, inspired by those who came before him.

A more complete quotation:

"To die is to be a counterfeit, for he is but the counterfeit of a man who hath not the life of a man; but to counterfeit dying, when a man thereby liveth, is to be no counterfeit, but the true and perfect image of life indeed. The better part of valor is discretion, in the which better part I have sav'd my life."

You are probably thinking of this quotation from Shakespeare's Henry IV Part 1, spoken by Falstaff: "The better part of valour is discretion; in the which better part I have saved my life."