Deuteronomy 15:17

Then thou shalt take an aul, and thrust it through his ear unto the door, and he shall be thy servant for ever. And also unto thy maidservant thou shalt do likewise.

Cross-references

Leviticus 25:39-42 forbids permanent enslavement of Israelites, contrasting with the voluntary permanent servitude allowed here.

Exodus 21:5 records the same law: the slave who chooses not to go free because of love for his master and family.

Exodus 21:6 describes the exact ear-piercing ritual for the willing slave, mirroring the procedure here.