Yeah, it's contained within Article 5 of the UEFA Champions' League (forgive the crappy formatting)
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To ensure the integrity of the UEFA club competitions (i.e. UEFA Champions League, UEFA Europa League and UEFA Europa Conference League), the following criteria apply:
No club participating in a UEFA club competition may, either directly or indirectly:
- hold or deal in the securities or shares of any other club participating in a UEFA club competition;
- be a member of any other club participating in a UEFA club competition;
- be involved in any capacity whatsoever in the management, administration and/or sporting performance of any other club participating in a UEFA club competition; or
- have any power whatsoever in the management, administration and/or sporting performance of any other club participating in a UEFA club competition.
No one may simultaneously be involved, either directly or indirectly, in any capacity whatsoever in the management, administration and/or sporting performance of more than one club participating in a UEFA club competition.
No individual or legal entity may have control or influence over more than one club participating in a UEFA club competition, such control or influence being defined in this context as:
- holding a majority of the shareholders’ voting rights
- having the right to appoint or remove a majority of the members of the administrative, management or supervisory body of the club;
- being a shareholder and alone controlling a majority of the shareholders’ voting rights pursuant to an agreement entered into with other shareholders of the club; or
- being able to exercise by any means a decisive influence in the decision-making of the club.
If two or more clubs fail to meet the criteria aimed at ensuring the integrity of the competition, only one of them may be admitted to a UEFA club competition, in accordance with the following criteria (applicable in descending order):
the club which qualifies on sporting merit for the most prestigious UEFA club competition (i.e., in descending order: UEFA Champions League, UEFA Europa League or UEFA Europa Conference League);
the club which was ranked highest in the domestic championship giving access to the relevant UEFA club competition;
- the club whose association is ranked highest in the access list (see Annex A).