Retrieving Source Documents

[LoGic142]

Is your feature request related to a problem? Please describe.
I wanted to know if we can get the source documents that the retriever brought back when used in a RAG pipeline.

Describe the solution you'd like
When I build a pipeline that ends with the OpenAIGenerator, I want to see the source documents retrieved by the retriever and used by the Generator to generate the response. If this exists in haystack, can someone please point me in the right direction?