CC Madhya 21.88

tasyāḥ pāre para-vyoma
tripād-bhūtaṁ sanātanam
amṛtaṁ śāśvataṁ nityam
anantaṁ paramaṁ padam
Synonyms: 
tasyāḥ pāre — on the other bank of the Virajā River; para-vyoma — the spiritual sky; tri-pāt-bhūtam — existing as three fourths of the opulence of the Supreme Lord; sanātanam — eternal; amṛtam — without deterioration; śāśvatam — without being subjected to the control of time; nityam — constantly existing; anantam — unlimited; paramam — supreme; padam — abode.
Translation: 
“ ‘Beyond the river Virajā is the spiritual nature, which is indestructible, eternal, inexhaustible and unlimited. It is the supreme abode, consisting of three fourths of the Lord’s opulences. It is known as paravyoma, the spiritual sky.”’
Purport: 

This is a verse from the Padma Purāṇa, quoted here by Lord Kṛṣṇa.