George Washington
At the end of his second term, Washington retired for personal and political reasons, fatigued and disgusted with personal attacks, and to assure a truly contested presidential election could be held. He did not feel bound to a two-term limit, but his retirement ultimately set precedent. The two-term limit to the presidency was formalized with the 1951 adoption of the Twenty-second Amendment to the United States Constitution. Washington is often credited with setting the principal of a two-term presidency, but it was Thomas Jefferson who first refused to run for a third term on political grounds.
Harry S. Truman
In 1951, the United States ratified the 22nd Amendment, making a president ineligible for election to a third term or for election to a second full term after serving more than two remaining years of a term of a previously elected president. The latter clause would have applied to Truman's situation in 1952 were it not for a grandfather clause excluding the amendment's application to the incumbent president.