SB 5.17.16
bhavani-nathaih stri-ganarbuda-sahasrair avarudhyamano bhagavatas catur-murter maha-purusasya turiyam tamasim murtim prakrtim atmanah sankarsana-samjñam atma-samadhi-rupena sannidhapyaitad abhigrnan bhava upadhavati.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In Ilavrta-varsa, Lord Siva is always encircled by ten billion maidservants of Goddess Durga, who minister to him. The quadruple expansion of the Supreme Lord is composed of Vasudeva, Pradyumna, Aniruddha and Sankarsana. Sankarsana, the fourth expansion, is certainly transcendental, but because His activities of destruction in the material world are in the mode of ignorance, He is known as tamasi, the Lord’s form in the mode of ignorance. Lord Siva knows that Sankarsana is the original cause of his own existence, and thus he always meditates upon Him in trance by chanting the following mantra.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sometimes we see a picture of Lord Siva engaged in meditation. This verse explains that Lord Siva is always meditating upon Lord Sankarsana in trance. Lord Siva is in charge of the destruction of the material world. Lord Brahma creates the material world, Lord Visnu maintains it, and Lord Siva destroys it. Because destruction is in the mode of ignorance, Lord Siva and his worshipable Deity, Sankarsana, are technically called tamasi. Lord Siva is the incarnation of tamo-guna. Since both Lord Siva and Sankarsana are always enlightened and situated in the transcendental position, they have nothing to do with the modes of material nature — goodness, passion and ignorance — but because their activities involve them with the mode of ignorance, they are sometimes called tamasi.