In the Śrīmad-Bhāgavatam, in the Tenth Canto, it is stated that, "This Rāma and Kṛṣṇa"—Rāma means Balarāma, and Kṛṣṇa—"They are the root of this material creation. And both of Them enters into each and every universe and maintains that."
Aṇḍāntara-stha-paramāṇu-cayāntara-stham
- (Bs 5.35).
This is mentioned in Śrīmad-Bhāgavatam.
- sṛṣṭi-hetu yei mūrti prapañce avatāre
- sei īśvara-mūrti 'avatāra' nāma dhare
- (CC Madhya 20.263)
"For material creation, the expansion of the Supreme Lord for the purpose of material creation is called incarnation. That is called avatāra."
- māyātīta paravyome sabāra avasthāna
- viśve avatari' dhare 'avatāra' nāma
- (CC Madhya 20.264)
"So all Viṣṇu expansion, they are in the spiritual world, and they all of them reside there. But when they come into this material world, it is called incarnation." Actually, avatāra . . . this Sanskrit name is avatāra. Avatāra means who comes down from the spiritual world. Avatāratī. Avatāratī means descends. Descends. They are not born of this material energy. They descend from the spiritual world.
- sei māyā avalokite śrī-saṅkarṣaṇa
- puruṣa-rūpe avatīrṇa ha-ilā prathama
- (CC Madhya 20.265)
Now, puruṣa-rūpe, the first incarnation for this material creation, the Mahā-Viṣṇu, sa aikṣata. Simply by His glance, this material world begins its activity.
- sei māyā avalokite sri-saṅkarṣaṇa
- puruṣa-rūpa avatīrṇa ha-ilā prathama
- (CC Madhya 20.265)
- jagṛhe pauruṣaṁ rūpaṁ
- bhagavān mahad-ādibhiḥ
- sambhūtaṁ ṣoḍaśa-kalam
- ādau loka-sisṛkṣayā
- (CC Madhya 20.266)
Ādau loka-sisṛkṣayā: "Just before the creation the Mahā-Viṣṇu, He takes His incarnation, and He puts His glance on the material energy, and the creation begins."