Exodus 21:6

21:6 then his master must bring him to the judges, and he will bring him to the door or the doorposts, and his master will pierce his ear with an awl, and he shall serve him forever.

Deuteronomy 15:17

15:17 you shall take an awl and pierce a hole through his ear to the door. Then he will become your servant permanently (this applies to your female servant as well).