Negative commandment · Lo Ta’aseh #292 of 613
Not to execute a person liable for execution before he stands trial
שלא להרוג מחוייב הריגה קודם שיעמוד בדין
The verse · Bamidbar 35:12
וְהָיוּ לָכֶם הֶעָרִים לְמִקְלָט מִגֹּאֵל וְלֹא יָמוּת הָרֹצֵחַ עַד־עׇמְדוֹ לִפְנֵי הָעֵדָה לַמִּשְׁפָּט
“And they shall be to you cities for refuge from the avenger; that the manslayer die not, until he stand before the congregation in judgment.”
The Rambam’s words
Not to execute a person liable for execution before he stands trial, as [Numbers 35:12] states: "The murderer shall not die [until he stands before the congregation]."
שלא להרוג מחוייב הריגה קודם שיעמוד בדין, שנאמר "וְלֹא יָמוּת הָרוֹצֵחַ עַד עָמְדוֹ לִפְנֵי הָעֵדָה" (במדבר לה, יב).
Why · Sefer HaChinuch #409
To not kill someone liable before he is brought to justice
“It is from the roots of the commandment [that] since the matter of capital punishment is a very weighty thing (Rosh Hashanah 26a) which requires the greatest of precision, and the community has been commanded to save the accused with everything that is fit to save him for his sake — not that they pervert the judgment in order to save him, G-d forbid, and like they, may their memory be blessed, expounded (Rosh Hashanah 26a, connecting the two phrases in Numbers 35:24-25), “And the community shall judge, And the community shall save,” meaning to say that they need to search for his merit and if he has a merit, they should save him, and if not, he should be killed — therefore, we were warned that the judgment should at the very least be brought before the court.”
Who & when
“And this commandment is practiced by males and females at all times, as we are commanded not to kill any sinner — and even though we saw him doing an act that would make him liable for the death penalty in court.”
Where in the Mishneh Torah
Hilchot Murderer and the Preservation of Life הלכות רוצח ושמירת נפש