Trung Bộ Kinh (Majjhima Nikaya) 2:146
SUJATOEvampi kho, bhikkhave, bhikkhu dhammesu dhammānupassī viharati chasu ajjhattikabāhiresu āyatanesu.
That’s how a mendicant meditates by observing an aspect of principles with respect to the six interior and exterior sense fields.