Obligatory wikipedia link.

The phrase "first, do no harm" is often, incorrectly, attributed to the oath.

On the other hand:

The origin of the phrase is uncertain. The Hippocratic Oath includes the promise "to abstain from doing harm" but not the precise phrase. Perhaps the closest approximation in the Hippocratic Corpus is in Epidemics: "The physician must...have two special objects in view with regard to disease, namely, to do good or to do no harm".

This was from the article on first, do no harm.