SharePoint 2013: Group Search Results by Site Name

Question

I have added a Search box and a Search Results web part with the query which retrieves documents from different subsites.

Now, what I want is to group this documents by using Site Name and show the documents below corresponding site label like below:

Site 1:

1. Document 1

2. Document 2

Site 2:

1. Document 1

2. Document 2

and so on.

OOTB solution will be preferred(not necessary that solution should be OOTB only).

Any help will be much apperciated.