מאג'ג'הימה ניקאיה 2:137
SUJATOPuna caparaṁ, bhikkhave, bhikkhu dhammesu dhammānupassī viharati chasu ajjhattikabāhiresu āyatanesu.
Furthermore, a mendicant meditates by observing an aspect of principles with respect to the six interior and exterior sense fields.