A reference is probably in one of the three original commentaries included in the Tipitaka. I will add that when I have time to look for it.
But “hadaya” means “heart”. “Hadaya vatthu” is the seat of mind that is located (on the gandhabba or the mental body) overlapping the physical heart.
– There is communication between the brain and hadaya vatthu (and the physical heart too). As Lvalio’s post says, modern science is beginning to realize that connection, even though they have no idea about hadaya vatthu.