Obtaining bearer token for Sharepoint REST API calls

Our scenario is following:

1. 
   

   User logs in through SSO (using Azure AD) to our website.

   
   


2. 
   

   On the site we have a page that will show links to some documents from SharePoint.

   
   


3. 
   

   The page will query SharePoint 2013 on prem through its REST API to get a list of those documents.

   
   

My problem is obtaining the Bearer token that I would use for my REST API calls. http://ift.tt/1Gbbswc

I would very much appreciate a code example on how to obtain the Bearer token.