Śrīmad-Bhāgavatam · Text 10.12.37
Who is speaking and story frame
एतत् कौमारजं कर्म हरेरात्माहिमोक्षणम् ।
मृत्यो: पौगण्डके बाला दृष्ट्वोचुर्विस्मिता व्रजे ॥ ३७ ॥
etat kaumārajaṁ karma
harer ātmāhi-mokṣaṇam
mṛtyoḥ paugaṇḍake bālā
dṛṣṭvocur vismitā vraje
This incident of Kṛṣṇa’s saving Himself and His associates from death and of giving deliverance to Aghāsura, who had assumed the form of a python, took place when Kṛṣṇa was five years old. It was disclosed in Vrajabhūmi after one year, as if it had taken place on that very day.
Read Śrīla Prabhupāda's purport on Vedabase.io