MALCOLM: Be this the whetstone of your sword. Let grief
Convert to anger. Blunt not the heart; enrage it.

William Shakespeare (1564-1616) English dramatist and poet

Macbeth, Act 4, sc. 3, l. 268ff (4.3.268-269) (1606)
    (Source)

To Macduff, after Macbeth's killers have murdered Macduff's family.