Srimad Bhagavatam Canto 05, Chapter 25, Text 01

SB 5.25.1

sri-suka uvaca
tasya mula-dese trimsad-yojana-sahasrantara aste ya vai kala bhagavatas tamasi samakhyatananta iti satvatiya drastr-drsyayoh sankarsanam aham ity abhimana-laksanam yam sankarsanam ity acaksate.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
Sri Sukadeva Gosvami said to Maharaja Pariksit: My dear King, approximately 240,000 miles beneath the planet Patala lives another incarnation of the Supreme Personality of Godhead. He is the expansion of Lord Visnu known as Lord Ananta or Lord Sankarsana. He is always in the transcendental position, but because He is worshiped by Lord Siva, the deity of tamo-guna or darkness, He is sometimes called tamasi. Lord Ananta is the predominating Deity of the material mode of ignorance as well as the false ego of all conditioned souls. When a conditioned living being thinks, “I am the enjoyer, and this world is meant to be enjoyed by me,” this conception of life is dictated to him by Sankarsana. Thus the mundane conditioned soul thinks himself the Supreme Lord.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
There is a class of men akin to Mayavadi philosophers who misinterpret the aham brahmasmi and so ’ham Vedic mantras to mean, “I am the Supreme Brahman” and “I am identical with the Lord.” This kind of false conception, in which one thinks himself the supreme enjoyer, is a kind of illusion. It is described elsewhere in Srimad-Bhagavatam (5.5.8): janasya moho ’yam aham mameti. As explained in the above verse, Lord Sankarsana is the predominating Deity of this false conception. Krsna confirms this in Bhagavad-gita (15.15):
sarvasya caham hrdi sannivisto
 mattah smrtir jñanam apohanam ca
“I am seated in everyone’s heart, and from Me come remembrance, knowledge and forgetfulness.” The Lord is situated in everyone’s heart as Sankarsana, and when a demon thinks himself one with the Supreme Lord, the Lord keeps him in that darkness. Although such a demoniac living entity is only an insignificant part of the Supreme Lord, he forgets his true position and thinks he is the Supreme Lord. Because this forgetfulness is created by Sankarsana, He is sometimes called tamasi. The name tamasi does not indicate that He has a material body. He is always transcendental, but because He is the Supersoul of Lord Siva, who must perform tamasic activities, Sankarsana is sometimes called tamasi.
Srimad Bhagavatam Canto 05, Chapter 25, Text 02