Digha Nikaya 2:30
SUJATOIti ajjhattaṁ vā kāye kāyānupassī viharati, bahiddhā vā kāye kāyānupassī viharati, ajjhattabahiddhā vā kāye kāyānupassī viharati.
And so they meditate observing an aspect of the body internally, externally, and both internally and externally.