Doubt thou the stars are fire, Doubt that the sun doth move, Doubt truth to be a liar, But never doubt I love.
Source documented
From William Shakespeare’s Hamlet, Act 2, Scene 2. In a letter to Ophelia, Hamlet declares that she may question even seemingly certain truths, but should never doubt his love. Polonius reads the letter to the King and Queen. Verified against the Folger Shakespeare Library’s text.