SSRS connection string Encryption
When you setup a data source, you can specify components of the connection string directly.

Simply add the following to the connection string:

Encrypt=True;TrustServerCertificate=True

That will force encryption between the SQL Server and the SSRS reporting server for the given data source.  Any report that uses that data source will obtain its data via an encrypted channel.  The TrustServerCertificate=True tells the client (SSRS) to trust the server certificate, even if it's a self-generated, or untrusted, certificate.