SQL Agent VS Linked Server

I have setup SQL Agent Job to run store procedures. I have linked server connection for source. When i run job i m getting following error.

 "Invalid connection string attribute". [SQLSTATE 01000] (Message 7412).

and

EXECUTE AS USER' failed for the requested user.

Can somebody let me know what credentials do we need to have for linked server and SQL Agent job to run.

Thanks for help.