“A sudden thought strikes me,—let us swear an eternal friendship.”
The Rovers, Act i, Sc. 1, reported in Bartlett's Familiar Quotations, 10th ed. (1919). Compare: "Let us embrace, and from this very moment vow an eternal misery together", Thomas Otway, The Orphan, Act iv., Sc. 2.; "My fair one, let us swear an eternal friendship", Jean Baptiste Molière, Le Bourgeois Gentilhomme (published c. 1871), act iv. sc. 1.