TT #49 · Parashas Noach · Fourth Aliyah · Passuk 9:5
But for your own life-blood I will require a reckoning, from every beast I will require it, and from man; from every man, his brother, I will require a reckoning for the life of man.
Torah Temimah points to R’ Elazar (Bava Kama 91b) — prohibition of self-injury.
E-Daf image — Bava Kama 91b (size=3)
“For your lives I will require” — R’ Elazar says: From your own lives I will require your blood; from here [we learn] that a person is not permitted to injure himself.
Custom (prohibition of self-harm derived from “le-nafshoteichem”).
A person may not willfully injure his own body; Hashem demands a reckoning even for self-inflicted harm.