This is the Maastricht treaty (1992):

Article 105a

l. The ECB shall have the exclusive right to authorize the issue of banknotes within the Community. The ECB and the national central banks may issue such notes. The banknotes issued by the ECB and the national central banks shall be the only such notes to have the status of legal tender within the Community.

2. Member States may issue coins subject to approval by the ECB of the volume of the issue. The Council may, acting in accordance with the procedure referred to in Article 189c and after consulting the ECB, adopt measures to harmonize the denominations and technical speci/ications of all coins intended for circulation to the extent necessary to permit their smooth circulation within the Community.

Article 128

The European Central Bank shall have the exclusive right to authorise the issue of euro banknotes within the Union. The European Central Bank and the national central banks may issue such notes. The banknotes issued by the European Central Bank and the national central banks shall be the only such notes to have the status of legal tender within the Union.

Member States may issue euro coins subject to approval by the European Central Bank of the volume of the issue. The Council, on a proposal from the Commission and after consulting the European Parliament and the European Central Bank, may, adopt measures to harmonise the denominations and technical specifications of all coins intended for circulation to the extent necessary to permit their smooth circulation within the Union.