Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 2.5.19

2.5.19

idaṃ vai tanmadhu dadhyaṅṅātharvaṇo'śvibhyāmuvāca tadetadṛṣiḥ paśyannavocadrūpam̐ rūpaṃpratirūpo babhūva tadasya rūpaṃ praticakṣaṇāya | indro māyābhiḥ pururūpa īyateyuktā hyasya harayaḥ śatā daśeti | ayaṃ vai harayo'yaṃ vai daśa ca sahasrāṇi bahūni cānantāni ca tadetadbrahmāpūrvamanaparamanantaramabāhyamayamātmā brahma sarvānubhūrityanuśāsanam

Verily Dadhyak Âtharvana proclaimed this honey to the two Asvins, and a Rishi, seeing this, said (Rv. VI, 47, 18):
'He (the Lord) became like unto every form, and this is meant to reveal the (true) form of him (the Âtman). Indra (the Lord) appears multiform through the Mâyâs (appearances), for his horses (senses) are yoked, hundreds and ten.'
This (Âtman) is the horses, this (Âtman) is the ten, and the thousands, many and endless. This is the Brahman, without cause and without effect, without anything inside or outside; this Self is Brahman, omnipresent and omniscient. This is the teaching (of the Upanishads).

2.5.19.p1

itidvitīyādhyāye pañcamaṃ brāhmaṇam