Mind in the heart was a common Greek view, not specific to Epicurus, see Why did ancient Greeks think of "heart as seat of soul"? Epicurus's works are not extant, except for three letters on other topics included by Diogenes Laertius. So, alas, there are no direct quotes. But Lucretius's De Rerum Natura is considered a faithful exposition of Epicurean views, and he discusses soul's function and location at length in Book 3, 3.190ff:
Now, I claim mind and soul are held united
and together form a single nature,
but the main one, which, as it were, has power
in the entire body, is our judgment,
which we call the mind or understanding,
fixed in place in the mid-part of the chest.
For here throb fear and terror. Soothing joys
move round this region, too. And therefore here
are mind and understanding. Of the soul,
all other parts, dispersed through the whole body,
obey and are moved in accordance with
the will and inclination of the mind.