Replacements (Law 3.4–3.10)

A player who has been replaced may not return to the field except to replace a player with a blood injury (Law 3.5, temporary blood replacement) or as a front-row replacement when no other suitably trained front-row player is available (Law 3.7). ...

Source: Law 3.5, Law 3.7, Law 3.6, Law 3.9

Send this to your friend — argument over.