מאג'ג'הימה ניקאיה 3:108
SUJATOYasmiṁ samaye, bhikkhave, bhikkhu kāye kāyānupassī viharati ātāpī sampajāno satimā vineyya loke abhijjhādomanassaṁ, upaṭṭhitāssa tasmiṁ samaye sati hoti asammuṭṭhā.
Whenever a mendicant meditates by observing an aspect of the body, at that time their mindfulness is established and lucid.