The 22nd Amendment to the U.S. Constitution, ratified February 27, 1951
The 22nd Amendment—ratified in 1951—codified the two-term tradition for the Presidency. George Washington began the tradition by retiring as President after two terms. The tradition was tested in 1880 and 1912, but it remained unbroken until Democratic President Franklin Delano Roosevelt won a third and fourth term in the White House in 1940 and 1944. In 1947, Congress proposed the 22nd Amendment amid concerns that without limits, the Presidency could become a dictatorship which lasted a lifetime.
General Records of the U.S. Government