You don't want me. You want the office of the president, and that doesn't belong to me. It belongs to the American people and it's not for sale.
Whether he ever used those exact words is debatable but he did express the sentiment in his 1960 book, Mr. Citizen:
I turned down all of those offers. I knew that they were not interested in hiring Harry Truman, the person, but what they wanted to hire was the former President of the United States. I could never lend myself to any transaction, however respectable, that would commercialize on the prestige and the dignity of the office of the Presidency.
While comparing that example to the one currently being presented by Pay-the-Bill(s) Clinton, it should be noted that when Mr. Truman declined the numerous lucrative proposals that were presented to him he was under considerable financial pressures. He left the presidency before any adequate pension was provided to former holders of the office.