Digha Nikaya 2:107
SUJATOIti ajjhattaṁ vā citte cittānupassī viharati, bahiddhā vā citte cittānupassī viharati, ajjhattabahiddhā vā citte cittānupassī viharati.
And so they meditate observing an aspect of the mind internally, externally, and both internally and externally.