SB 3.33.2

devahūtir uvāca
athāpy ajo ’ntaḥ-salile śayānaṁ
 bhūtendriyārthātma-mayaṁ vapus te
guṇa-pravāhaṁ sad-aśeṣa-bījaṁ
 dadhyau svayaṁ yaj-jaṭharābja-jātaḥ
Synonyms: 
devahūtiḥ uvāca — Devahūti said; atha api — moreover; ajaḥ — Lord Brahmā; antaḥ-salile — in the water; śayānam — lying; bhūta — the material elements; indriya — the senses; artha — the sense objects; ātma — the mind; mayam — pervaded by; vapuḥ — body; te — Your; guṇa-pravāham — the source of the stream of the three modes of material nature; sat — manifest; aśeṣa — of all; bījam — the seed; dadhyau — meditated upon; svayam — himself; yat — of whom; jaṭhara — from the abdomen; abja — from the lotus flower; jātaḥ — born.
Translation: 
Devahūti said: Brahmā is said to be unborn because he takes birth from the lotus flower which grows from Your abdomen while You lie in the ocean at the bottom of the universe. But even Brahmā simply meditated upon You, whose body is the source of unlimited universes.
Purport: 

Brahmā is also named Aja, “he who is unborn.” Whenever we think of someone’s birth, there must be a material father and mother, for thus one is born. But Brahmā, being the first living creature within this universe, was born directly from the body of the Supreme Personality of Godhead who is known as Garbhodakaśāyī Viṣṇu, the Viṣṇu form lying down in the ocean at the bottom of the universe. Devahūti wanted to impress upon the Lord that when Brahmā wants to see Him, he has to meditate upon Him. “You are the seed of all creation,” she said. “Although Brahmā was directly born from You, he still has to perform many years of meditation, and even then he cannot see You directly, face to face. Your body is lying within the vast water at the bottom of the universe, and thus You are known as Garbhodakaśāyī Viṣṇu.”

The nature of the Lord’s gigantic body is also explained in this verse. That body is transcendental, untouched by matter. Since the material manifestation has come from His body, His body therefore existed before the material creation. The conclusion is that the transcendental body of Viṣṇu is not made of material elements. The body of Viṣṇu is the source of all other living entities, as well as the material nature, which is also supposed to be the energy of that Supreme Personality of Godhead. Devahūti said, “You are the background of the material manifestation and all created energy; therefore Your delivering me from the clutches of māyā by explaining the system of Sāṅkhya philosophy is not so astonishing. But Your being born from my abdomen is certainly wonderful because although You are the source of all creation, You have so kindly taken birth as my child. That is most wonderful. Your body is the source of all the universe, and still You put Your body within the abdomen of a common woman like me. To me, that is most astonishing.”