The oldest of the Upanishad texts originated from the 6th century B.C. – including the Chandogya Upanishads. This extract from chapter VIII.6 is perhaps the first mention of the Nadis.

Upanishads

Chhandogya-Upanishad

Sentence 6

tadesha shlokah |"
shatam cha-ika cha hridayasya nadyas-tasam murdhanam-abhinihsrita-ika |"
tay[a]-{o}[u]dhrvam-ayann-anritatvam-eti vishvann-anya utkramane bhavanty-utkramane bhavanti ||6||

तदेष श्लोकः ।"
शतं चैका च हृदयस्य नाड्यस्तासां मूर्धानमभिनिःसृतैका ।"
तय्[आ]{ओ}[उ]ध्र्वमायन्नमृतत्वमेति विष्वङ्ङन्या उत्क्रमणे भवन्त्युत्क्रमणे भवन्ति ॥६॥

tadeṣa ślokaḥ ।"
śataṁ ca-ikā ca hr̥dayasya nāḍyas-tāsāṁ mūrdhānam-abhiniḥsr̥ta-ikā ।"
tay[ā]-{o}[u]dhrvam-āyann-amr̥tatvam-eti viṣvaṅṅ-anyā utkramaṇe bhavanty-utkramaṇe bhavanti ॥6॥

On this a holy mantra: |
There are 101 energy channels (Nadis) of the heart existing. One of them is directed to the skullcap. |
Through this one, reaching upwards, immortality (Amrita) can be achieved. All other channels are going to various directions - they are doors to death, exitus doorways. ||6||

tat (तत्, tat) = whose, reffering to before
eshah (एषः, eṣaḥ) = this
shlokah (श्लोकः, ślokaḥ) = verse, mantra
bhavati (भवति, bhavati) = sublime, sacred

shatam (शतं, śataṁ) = 100
cha (च, ca) = and
eka (एका, ekā) = 1
shatam cha-ika (शतम् चैका, śatam ca-ikā) = 101
cha (च, ca) = and
hridayasya (हृदयस्य, hr̥dayasya) = of the heart
nadyah (नाड्यः, nāḍyaḥ) = Nadi, subtle energy channel
tasam (तासां, tāsāṁ) = from those
murdhanam (मूर्धनम्, mūrdhanam) = the skullcap, fontanelle
abhinihsrita (अभिनिःसृता, abhiniḥsr̥tā) = is going towards, exceeding through

taya (तया, tayā) = through this
urdhvam (ऊर्ध्वम्, ūrdhvam) = upwards
ayan (आयन्, āyan) = going
anritatvam (अमृतत्वम्, amr̥tatvam) = nectar of immortality, immortality
eti (एति, eti) = 1
vishvam (विश्वम्, viśvam) = going to different directions
anyah (अन्यः, anyaḥ) = the others (nadis)
utkramane (उत्क्रमणे, utkramaṇe) = of death, the abandonment of the body for
reincarnation bhavanti (भवन्ति, bhavanti) = will doors
utkramane bhavanti (उत्क्रमणे भवन्ति, utkramaṇe bhavanti) = gemination for affirmation