BṚHADĀRAṆYAKA UPANISHAD · 2.5.19

इदं वै तन् मधु दध्यङ्ङ् आथर्वणो ऽश्विभ्याम् उवाच ।
तद् एतद् ऋषिः पश्यन्न् अवोचत् -- रूपंरूपं प्रतिरूपो बभूव तद् अस्य रूपं प्रतिचक्षणाय ।
इन्द्रो मायाभिः पुरुरूप ईयते युक्ता ह्य् अस्य हरयः शता दशेति ।
अयं वै हरयो ऽयं वै दश च सहस्रणि बहूनि चानन्तानि च ।
तद् एतद् ब्रह्मापूर्वम् अनपरम् अनन्तरम् अबाह्यम् ।
अयम् आत्मा ब्रह्म सर्वानुभूः ।
इत्य् अनुशासनम् ॥

idaṃ vai tan madhu dadhyaṅṅ ātharvaṇo ’śvibhyām uvāca ।
tad etad ṛṣiḥ paśyann avocat -- rūpaṃrūpaṃ pratirūpo babhūva tad asya rūpaṃ praticakṣaṇāya ।
indro māyābhiḥ pururūpa īyate yuktā hy asya harayaḥ śatā daśeti ।
ayaṃ vai harayo ’yaṃ vai daśa ca sahasraṇi bahūni cānantāni ca ।
tad etad brahmāpūrvam anaparam anantaram abāhyam ।
ayam ātmā brahma sarvānubhūḥ ।
ity anuśāsanam ॥

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).

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)