Negative commandment · Lo Ta’aseh #297 of 613
Not to neglect [a person] in mortal danger
שלא לעמוד על הדם
The verse · Vayikra 19:16
לֹא־תֵלֵךְ רָכִיל בְּעַמֶּיךָ לֹא תַעֲמֹד עַל־דַּם רֵעֶךָ אֲנִי ה׳
“Thou shalt not go up and down as a talebearer among thy people: neither shalt thou stand aside when mischief befalls thy neighbour: I am the Lord.”
The Rambam’s words
Not to neglect [a person] in mortal danger, as [Leviticus 19:16] states: "Do not stand still in the face of mortal danger."
שלא לעמוד על הדם, שנאמר "לֹא תַעֲמֹד עַל דַּם רֵעֶךָ" (ויקרא יט, טז).
Why · Sefer HaChinuch #237
To not stand over the blood of neighbors
“The root of this commandment is well-known — as [just] like one will save his fellow, so [too,] will his fellow save him. And the world will be inhabited like this, and G-d desires its habitation, as ‘He created it to be inhabited.’ And the laws of this commandment have already been elucidated in Tractate Sanhedrin.”
Who & when
“And it is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Murderer and the Preservation of Life הלכות רוצח ושמירת נפש