Single observer with multiple observable

I would like to know what would be the implementation with respect to observer design pattern. Like generally we maintain a list of observers in Observable and from its notify we call the update method of the observers.

Now what would be the design in this case? Should i maintain a list of observable in the observers? But then in the update method how i would know which observable has called the update on this observer.

Please share your idea on this.

Thanks,