There is no current report to see referral sources but this information can be found by creating a segment.
Navigate to Marketing/Segments/New segment
Create a rule for Client/Creation date and then select the time frame you are interested in.
Clicking client count will tell you how many clients will be listed in the results.
Save the segment and you will be returned to the segment list.
Click the Options button to the right of your newly created segment and download as csv.
This will give you a list of all the clients created in the date you specified with a column for referral source.