I shall not again be a candidate for the Presidency... I shall be 65 on the 23rd April next, and I had determined upon my line of life from the remnant of days with which a kind Providence might bless me, discarding every idea of the Presidency... I can now leave public life, I trust, with credit. Should I become President, the case may be very different, after I shall have worn myself out with the toil and anxiety of the office.

Responding to suggestions that he run for President in 1856, as quoted at wheatland.org.