Negative commandment · Lo Ta’aseh #288 of 613
Not to render a decision based on the testimony of a single witness
שלא לכרות הדין על פי עד אחד
The verse · Devarim 19:15
לֹא־יָקוּם עֵד אֶחָד בְּאִישׁ לְכׇל־עָוֺן וּלְכׇל־חַטָּאת בְּכׇל־חֵטְא אֲשֶׁר יֶחֱטָא עַל־פִּי שְׁנֵי עֵדִים אוֹ עַל־פִּי שְׁלֹשָׁה־עֵדִים יָקוּם דָּבָר
“One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he may commit: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.”
The Rambam’s words
Not to render a decision based on the testimony of a single witness, as [Deuteronomy 19:15] states: "A single witness shall not rise up against a person."
שלא לכרות הדין על פי עד אחד, שנאמר "לֹא יָקוּם עֵד אֶחָד בְּאִישׁ" (דברים יט, טו).
Why · Sefer HaChinuch #523
To not establish a matter of testimony with one witness
“It is from the roots of the commandment [that it is] since the impulse of a man’s heart is evil, and sometimes a passion comes to his heart against his fellow; and even if he is an extremely proper man, he is not saved from sometimes sinning.”
Who & when
“And this commandment is practiced regarding the matter of monetary cases that are practiced today in every place and at all times by males, since it is for them to administer justice.”
Where in the Mishneh Torah
Hilchot Testimony הלכות עדות