Manava Dharmashastra 1.92

1.92

āpādāntānmedhyaḥ puruṣaḥ | tasya nābherūrdhvamatiśayena medhyam | tato'pi mukham | etacca svayameva jagatkāraṇapuruṣeṇoktam |
tataḥ kimata āha-

Man is described as purer above his naval; hence the Self-existent One has declared the mouth to be his purest part. (92)

Medhātithi’s commentary (manubhāṣya):
Down to the very sole of his feet, Man is pure; that part of his body which is ‘above the naval’ is extremely pure; and purer even than that is his mouth. This has been declared by that person himself who is the creator of the world. (92)