Leviticus 19:28

There is a scholarly paper on it here: https://www.jstor.org/stable/23496450
Abstract:
Lev 19:28 prohibits tattooing, but no reason for the prohibition is given. Since it appears in a context of pagan mourning practices (Lev 19:27,28) it is assumed that the reason for the prohibition lay in its association with such mourning practices. In this paper we explore the broader context of the law in biblical times, and how it was understood in subsequent rabbinic times. We propose that in the biblical period the prohibition was associated with the marking of slaves, and that in the subsequent rabbinic period it was associated with paganism.