Artist: Muddy Waters
Song: Mannish Boy

Standard Tuning

h = hammer on
x = muted string

Main Riff:

e|----------------------------------x-|
B|----------------------------------x-|
G|-------------7------------5-h-6---x-|
D|--------7---------7---------------x-|
A|--------0---------0---------------x-|
E|----------------------------------x-|

       (Palm Mute after hammer on)

This is how the riff was given in Total Guitar but I prefer my version, maybe I'm biased:

e|-----------------------------------|
B|-----------------------------------|
G|-------------7------------5-h-6----|
D|--------7----7----7----------------|
A|--------0---------0----------------|
E|-----------------------------------|