मज्झिम निकाय 2:165
SUJATOIti ajjhattaṁ vā dhammesu dhammānupassī viharati, bahiddhā vā dhammesu dhammānupassī viharati, ajjhattabahiddhā vā dhammesu dhammānupassī viharati;
And so they meditate observing an aspect of principles internally, externally, and both internally and externally.