HORATIO: What is it, my lord? We will do it.
HAMLET: Never let it be known what you have seen tonight.
HORATIO AND MARCELLUS: My lord, we will not.
HAMLET: No. You must swear it.
HORATIO: Truly, my lord, I will not.
MARCELLUS: Nor I, my lord, truly.
HAMLET: Swear by my sword.
MARCELLUS: We have sworn already, my lord.
HAMLET: Indeed—now do it upon my sword.