Majjhima Nikaya 2:39
SUJATOPuna caparaṁ, bhikkhave, bhikkhu imameva kāyaṁ uddhaṁ pādatalā, adho kesamatthakā, tacapariyantaṁ pūraṁ nānappakārassa asucino paccavekkhati:
Furthermore, a mendicant examines their own body, up from the soles of the feet and down from the tips of the hairs, wrapped in skin and full of many kinds of filth.