Heart of Hearts

Any ideas on where this phrase comes from?

Obviously heart means centre and so it is from the centre of my heart.

But where did the phrase originate from?

Any clues?

thanks,

Alex.

Brewer's Dictionary of Phrase and Fable:

Heart of Hearts (In one's). In one's inmost conviction. The heart is often referred to as a second self. Shakespeare speaks of the "neck of the heart" (Merchant of Venice, ii. 2); "the middle of the heart" (Cymbeline, i. 7). The heart of the heart is to the same effect.

Is anyone aware of earlier sources?