There is no specific regulation requiring or allowing it; this is not the purpose of regulation. However, the CAMO is responsible for ensuring the continuing airworthiness of aircraft and is obliged to ensure adequate maintenance standards on the part of contracted maintenance organisations. It is required to do this through scheduled and, if considered necessary, unscheduled audits. I hope that helps with your understanding?

Exactly what David says and in additional to this: It should be a part of the contract to have a provision for (unscheduled) audits.
Ref. Appendix IV to AMC1 CAMO.A.315(c) - Contracted maintenance, item 2.5.

You could also refer to point 2.8.5 of the proposed CAME. This is linked to point 6 of CAMO.A.315(b) which states that the CAMO shall "supervise activities, and coordinate related decisions to ensure that any maintenance is carried out properly and is appropriately released".

Thank you so much David, Vincent and Roger! Much appreciated.

Dear Dilek,
Hope this reference help you,
Appendix XI to AMC M.A.708(c) — Contracted maintenance
2.5. Quality monitoring
The terms of the contract should include a provision allowing the CAMO to perform a quality surveillance (including audits) of the maintenance organisation. The maintenance contract should specify how the results of the quality surveillance are taken into account by the maintenance organisation..