Chāndogya Upaniṣad 1.3.7

1.3.7

dyaur evot \
antarikṣaṃ gīḥ \
pṛthivī tham \
āditya evot \
vāyur gīḥ \
agnis tham \
sāmaveda evot \
yajurvedo gīḥ \
ṛgvedas tham \
dugdhe asmai vāg dohaṃ yo vāco dohaḥ \
annavān annādo bhavati \
ya etāny evaṃ vidvān udgīthākṣarāṇy upāsta ud_ gī tha iti \\103.7\

7. ud is heaven; gī is atmosphere; tha is the earth. ud is the sun; gī is wind; tha is fire. ud is Sāma-Veda; gī is Yajur-Veda; tha is Rig-Veda. Speech yields milk—that is, the milk of speech itself—for him, he becomes rich in food, an eater of food, who knows and reverences these syllables of the Udgītha thus: ud, gī, tha.