Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 13

Sanskrit:

तं वटुं वामनं द‍ृष्ट्वा मोदमाना महर्षय: ।कर्माणि कारयामासु: पुरस्कृत्य प्रजापतिम् ॥ १३ ॥

ITRANS:

taṁ vaṭuṁ vāmanaṁ dṛṣṭvāmodamānā maharṣayaḥkarmāṇi kārayām āsuḥpuraskṛtya prajāpatim

Translation:

When the great sages saw the Lord as the brahmacārī-dwarf Vāmana, they were certainly very pleased. Thus they placed before them Kaśyapa Muni, the Prajāpati, and performed all the ritualistic ceremonies, such as the birthday ceremony.

Purport:

According to Vedic civilization, when a child is born in the family of a brāhmaṇa, the birthday ceremony, known as jāta-karma, is first performed, and then other ceremonies are also gradually performed. But when this vāmana-rūpa appeared in the form of a vaṭu, or brahmacārī, His sacred thread ceremony was also performed immediately.