Trường Bộ Kinh (Digha Nikaya) 2:145
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 …