Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 35-36
पीतप्रायस्य जननी सुतस्य रुचिरस्मितम् ।मुखं लालयती राजञ्जृम्भतो ददृशे इदम् ॥ ३५ ॥खं रोदसी ज्योतिरनीकमाशा:सूर्येन्दुवह्निश्वसनाम्बुधींश्च ।द्वीपान् नगांस्तद्दुहितृर्वनानिभूतानि यानि स्थिरजङ्गमानि? ॥ ३६ ॥
pīta-prāyasya jananīsutasya rucira-smitammukhaṁ lālayatī rājañjṛmbhato dadṛśe idam
O King Parīkṣit, when the child Kṛṣṇa was almost finished drinking His mother’s milk and mother Yaśodā was touching Him and looking at His beautiful, brilliantly smiling face, the baby yawned, and mother Yaśodā saw in His mouth the whole sky, the higher planetary system and the earth, the luminaries in all directions, the sun, the moon, fire, air, the seas, islands, mountains, rivers, forests, and all kinds of living entities, moving and nonmoving.
By the arrangement of Yoga-māyā, Kṛṣṇa’s pastimes with mother Yaśodā were all regarded as ordinary. So here was an opportunity for Kṛṣṇa to show His mother that the whole universe is situated within Him. In His small form, Kṛṣṇa was kind enough to show His mother the virāṭ-rūpa, the universal form, so that she could enjoy seeing what kind of child she had on her lap. The rivers have been mentioned here as the daughters of the mountains (nagāṁs tad-duhitṝḥ). It is the flowing of the rivers that makes big forests possible. There are living entities everywhere, some of them moving and some of them not moving. No place is vacant. This is a special feature of God’s creation.