Negative commandment · Lo Ta’aseh #298 of 613
Not to leave obstacles
שלא להניח מכשול
The verse · Devarim 22:8
כִּי תִבְנֶה בַּיִת חָדָשׁ וְעָשִׂיתָ מַעֲקֶה לְגַגֶּךָ וְלֹא־תָשִׂים דָּמִים בְּבֵיתֶךָ כִּי־יִפֹּל הַנֹּפֵל מִמֶּנּוּ
“When thou buildest a new house, then thou shalt make a parapet for thy roof, that thou bring not blood upon thy house, if any man fall from it.”
The Rambam’s words
Not to leave obstacles, as [Deuteronomy 22:8] states: "...lest you bring blood upon your house."
שלא להניח מכשול, שנאמר "וְלֹא תָשִׂים דָּמִים בְּבֵיתֶךָ" (דברים כב, ח).
Why · Sefer HaChinuch #547
To not leave a stumbling block
“It is from the roots of the commandment [that it is] since even though G-d, may He be blessed, supervises the details of people’s [lives] and knows all of their deeds, and [that] everything that happens to them — good or bad — is through His decree and His commandment according to their merit or their guilt, and like the matter that they, may their memory be blessed, said (Chullin 7b), “A man does not [so much as] bruise his finger below (i.e. on earth), unless it is announced about him from above (in Heaven)”; nonetheless a person must guard himself from the accidents that are customary in the world.”
Where in the Mishneh Torah
Hilchot Murderer and the Preservation of Life הלכות רוצח ושמירת נפש