Srimad Bhagavatam Canto 03, Chapter 13, Text 22

SB 3.13.22

drsto ’ngustha-siro-matrah
 ksanad ganda-sila-samah
api svid bhagavan esa
 yajño me khedayan manah
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
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, Visnu?
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Since Brahma 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 Visnu. The uncommon features symptomatic of the incarnation of Godhead can bewilder even the mind of Brahma.
Srimad Bhagavatam Canto 03, Chapter 13, Text 21
Srimad Bhagavatam Canto 03, Chapter 13, Text 23