Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.2.2

3.1.2.2

athottareṇa śālām pariśrayanti | tadudakumbhamupanidadhāti tannāpita upatiṣṭhate tatkeśaśmaśru ca vapate nakhāni ca nikṛntate 'sti vai puruṣasyāmedhyaṃ yatrāsyāpo nopatiṣṭhante keśaśmaśrau ca vā asya nakheṣu cāpo nopatiṣṭhante tadyatkeśaśmaśru ca vapate nakhāni ca nikṛntate medhyo bhūtvā dīkṣā iti

Thereupon they enclose a place north of the hall, and place a vessel of water in it. Beside this the barber takes up his position. He (the sacrificer) then shaves his hair and beard, and cuts his nails. For impure, indeed, is that part of man where water does not reach him. Now at the hair and beard, and at the nails the water does not reach him: hence when he shaves his hair and beard, and cuts his nails, he does so in order that he may be consecrated after becoming pure.