SB 10.54.44

eka eva paro hy ātmā
 sarveṣām api dehinām
nāneva gṛhyate mūḍhair
 yathā jyotir yathā nabhaḥ
Synonyms: 
ekaḥ — one; eva — only; paraḥ — the Supreme; hi — indeed; ātmā — Soul; sarveṣām — among all; api — and; dehinām — embodied beings; nānā — many; iva — as if; gṛhyate — is perceived; mūḍhaiḥ — by those who are bewildered; yathā — as; jyotiḥ — a celestial body; yathā — as; nabhaḥ — the sky.
Translation: 
Those who are bewildered perceive the one Supreme Soul, who resides in all embodied beings, as many, just as one may perceive the light in the sky, or the sky itself, as many.
Purport: 

The last line of this text, yathā jyotir yathā nabhaḥ, introduces two analogies in which we perceive one thing to be many. Jyotiḥ indicates the light of heavenly bodies such as the sun or moon. Although there is only one moon, we may see the moon reflected in pools, rivers, lakes and buckets of water. Then it would appear as if there were many moons, although there is only one. Similarly, we perceive a divine presence in each living being because the Supreme Lord is present everywhere, although He is one. The second analogy given here, yathā nabhaḥ, is that of the sky. If we have a row of sealed clay pots in a room, the sky, or air, is in each pot, although the sky itself is one.

The Śrīmad-Bhāgavatam (1.2.32) gives a similar analogy concerning fire and wood:

yathā hy avahito vahnir
 dāruṣv ekaḥ sva-yoniṣu
nāneva bhāti viśvātmā
 bhūteṣu ca tathā pumān

“The Lord, as the Supersoul, pervades all things, just as fire permeates wood, and so He appears to be of many varieties, though He is the absolute one without a second.”