Or HaChaim on Genesis 39:17/אור החיים על בראשית ל״ט:י״ז

1 ותדבר אליו..לאמור, She spoke to him…saying. The word לאמור here is meant as a reference to the testimony which the members of the household would be willing to supply. Potiphar's anger was not so much directed at Joseph but at the fact that his wife was able to reinforce her accusations by calling the members of the household as witnesses. This is why the Torah writes: "when his master heard the words of his wife which she spoke to him לאמור כדברים האלה." The Torah did not need to write more than: "when his master heard the words of his wife." We are meant to realise that Potiphar only became angry after לאמור, i.e. that there were independent witnesses supporting his wife's tale.

2לאמור כדברים האלה, to corroborate such words. Potiphar did not believe his wife. Since the accuser was his own wife, however, he had to make some gesture otherwise his wife would have been publicly discredited. This is why Potiphar did not discipline Joseph nor had him executed, the normal penalty for a slave who dared to aspire to the wife of his master.