Explanation:

- Option 3: Gray — John Chipman Gray defined precedent as: “A precedent covers everything said or done, which furnishes a rule for subsequent practice.”

- Option 1: Salmond — He described precedent as a judicial decision which contains a legal principle, but is more focused on binding force and doctrine rather than the above wording.

- Option 2: Keeton — Keeton wrote about judicial precedents, but did not use this exact definition.

- Option 4: Allen — Allen discussed the doctrine of precedent in depth, but again, this specific phrase is not attributed to him.