Relocation: What Law Requires Permission to Move Overseas With the Children

My question involves a child custody case from the State of: OR

a) 50/50 Custody.
b) Ex-wife would like to move with our children to Europe.

Question:
What specific law and § regulates that she needs to get permission from the father of the children to move to Europe?

Thank you so much for any little help,
appreciate it!

Relocation: What Law Requires Permission to Move Overseas With the Children