It's the Blues Brothers extended instrumental intro of an Otis Redding song called "I can't turn you loose". The Blues Brothers play it a little faster than the original. Also the Blues Brothers played the whole song in it's intierty usually (eventually after all the crowd banter was over). You can find it on a lot of Blues Brothers albums as they used it for their intro also. The original is of course on a bunch of Otis Redding comps.
Check out this site
http://www.bluesbrotherscentral.com/index.php
They have some free mp3's and a lot of blues brothers info and a few boots.