SB 3.8.15
tal loka-padmam sa u eva visnuh
pravivisat sarva-gunavabhasam
tasmin svayam vedamayo vidhata
svayambhuvam yam sma vadanti so ’bhut
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Into that universal lotus flower Lord Visnu personally entered as the Supersoul, and when it was thus impregnated with all the modes of material nature, the personality of Vedic wisdom, whom we call the self-born, was generated.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
This lotus flower is the universal virat form, or the gigantic form of the Lord in the material world. It becomes amalgamated in the Personality of Godhead Visnu, in His abdomen, at the time of dissolution, and it becomes manifest at the time of creation. This is due to Garbhodakasayi Visnu, who enters into each of the universes. In this form is the sum total of all the fruitive activities of the living entities conditioned by material nature, and the first of them, namely Brahma, or the controller of the universe, is generated from this lotus flower. This first-born living being, unlike all the others, has no material father, and thus he is called self-born, or svayambhu. He goes to sleep with Narayana at the time of devastation, and when there is another creation, he is born in this way. From this description we have the conception of three — the gross virat form, the subtle Hiranyagarbha and the material creative force, Brahma.