Benjamin Disraeli: The secret of success is constancy of purpose.

Source

speech at banquet of National Union of Conservative and Constitutional Associations, Benjamin Disraeli, speech at banquet of National Union of Conservative and Constitutional Associations, Crystal Palace, London (24 June 1872); Selected Speeches of the Right Honourable the Earl of Beaconsfield, ed. T. E. Kebbel, vol. 2, p. 535 (1882)