Chānd-Mül, 1879-841. FIFTH KHANDA
The udgitha is the pranava, the pranava is the udgitha. And as the udgitha is the sun, So is the pranava, for he (the sun) goes sounding Om.
Chānd-Śaṃ, 8th c. A.D.prāṇādityadṛṣṭiviśiṣṭasyodgīthasyopāsanamuktamevānūdya praṇavodgīthayorekatvaṃ kṛtvā tasminprāṇaraśmibhedaguṇaviśiṣṭadṛṣṭyākṣarasyopāsanamanekaputraphalamidānīṃ vaktavyamityārabhyate-atha khalu ya udgīthaḥ sa praṇavo bahvṛcānāṃ yaśca praṇavasteṣāṃ sa eva cchāndogya udgīthaśabdavācyaḥ | asau vā āditya udgītha eṣa praṇavaśabdavācyo ’pi sa eva bahvṛcānāṃ nānyaḥ | udgītha ādityaḥ katham | udgīthākhyamakṣaramomityetadeṣa hi yasmātsvarannuccārayannanekārthatvāddhātūnām | athavā svarangacchannaiti | ato ’sāvudgīthaḥ savitā || 1 ||
http://www2.hf.uio.no/common/apps/permlink/permlink.php?app=polyglotta&context=record&uid=cba1c894-8e41-11ee-937a-005056a97067