SB 8.18.12
yat tad vapur bhati vibhusanayudhair
avyakta-cid-vyaktam adharayad dharih
babhuva tenaiva sa vamano vatuh
sampasyator divya-gatir yatha natah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Lord appeared in His original form, with ornaments and weapons in His hands. Although this ever-existing form is not visible in the material world, He nonetheless appeared in this form. Then, in the presence of His father and mother, He assumed the form of Vamana, a brahmana-dwarf, a brahmacari, just like a theatrical actor.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The word natah is significant. An actor changes dress to play different parts, but is always the same man. Similarly, as described in the Brahma-samhita (5.33, 39), the Lord assumes many thousands and millions of forms (advaitam acyutam anadim ananta-rupam adyam purana-purusam). He is always present with innumerable incarnations (ramadi-murtisu kala-niyamena tisthan nanavataram akarod bhuvanesu kintu). Nonetheless, although He appears in various incarnations, they are not different from one another. He is the same person, with the same potency, the same eternity and the same spiritual existence, but He can simultaneously assume various forms. When Vamanadeva appeared from the womb of His mother, He appeared in the form of Narayana, with four hands equipped with the necessary symbolic weapons, and then immediately transformed Himself into a brahmacari (vatu). This means that His body is not material. One who thinks that the Supreme Lord assumes a material body is not intelligent. He has to learn more about the Lord’s position. As confirmed in Bhagavad-gita (4.9), janma karma ca me divyam evam yo vetti tattvatah. One has to understand the transcendental appearance of the Lord in His original transcendental body (sac-cid-ananda-vigraha).