You can get to the Leads By Referral Source Report by going to Reports > Leads > Leads By Referral Source.
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This will then bring you to the screen below:
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RED - Choose the start and stop date for the Report.

ORANGE - Choose the branches (if your company has multiple branches) here.

YELLOW - Don't forget to click "View" to see your report!

GREEN - These button will allow you to either Export to PDF or CSV file.