If you are using Active Directory (AD) user accounts then this is easy, you just have the datasource use Windows Integrated Security. If you are not using AD then I suspose you could probably create some code to map the users AD account to a SQL Server account and have it logon automagically. Otherwise you could use "Credentials Stored Securely on the server" where you use a single user name and password for the data source and control access to the reports by user using the security tab under each report or folder.