Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 22

Sanskrit:

द‍ृष्टोऽङ्गुष्ठशिरोमात्र: क्षणाद्‍गण्डशिलासम: ।अपि स्विद्भगवानेष यज्ञो मे खेदयन्मन: ॥ २२ ॥

ITRANS:

dṛṣṭo ’ṅguṣṭha-śiro-mātraḥkṣaṇād gaṇḍa-śilā-samaḥapi svid bhagavān eṣayajño me khedayan manaḥ

Translation:

First of all this boar was seen no bigger than the tip of a thumb, and within a moment He was as large as a stone. My mind is perturbed. Is He the Supreme Personality of Godhead, Viṣṇu?

Purport:

Since Brahmā is the supermost person in the universe and he had never before experienced such a form, he could guess that the wonderful appearance of the boar was an incarnation of Viṣṇu. The uncommon features symptomatic of the incarnation of Godhead can bewilder even the mind of Brahmā.