Satipathanna Sutta and contemplating mind externally?
Abhidhamma Vibhaṅga Khandhavibhaṅga:

Therein what is internal consciousness? That consciousness which, for this or that being, is personal, self-referable, one’s own, individual and is grasped (by craving and false view), (i.e.) eye consciousness, ear consciousness, nose consciousness, tongue consciousness, body consciousness, mind consciousness. This is called internal consciousness.
Therein what is external consciousness? That consciousness which, for this or that other being, for other persons is personal, self-referable, one’s own, individual and is grasped, (i.e.) eye consciousness, ear consciousness, nose consciousness, tongue consciousness, body consciousness, mind consciousness. This is called external consciousness.