Grant permissions on stored procedure

I created a user in SQL Server 2012 database and revoked all permissions given by the public role.

Then i granted exec permission on a stored procedure. The user can execute the procedure but cannot get the data it returns.

The procedure is in a schema1 and the tables from which it selects are in schema2.

If I add the user to db_datareader role it can read all data from all the tables in the database. I tried WITH EXECUTE AS OWNER but it didn't work.

How can I grant only the access to the given procedure and nothing else?