Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 48

Sanskrit:

अद्यानेन महाव्यालो यशोदानन्दसूनुना ।हतोऽविता वयं चास्मादिति बाला व्रजे जगु: ॥ ४८ ॥

ITRANS:

adyānena mahā-vyāloyaśodā-nanda-sūnunāhato ’vitā vayaṁ cāsmāditi bālā vraje jaguḥ

Translation:

As the cowherd boys reached the village of Vraja, they sang, “Today Kṛṣṇa saved us by killing a great serpent!” Some of the boys described Kṛṣṇa as the son of Yaśodā, and others as the son of Nanda Mahārāja.

Purport:

Actually, Lord Kṛṣṇa had killed the demon Aghāsura one year earlier, but the boys, bewildered by the mystic potency of Brahmā for one year, did not notice the time passing and thus thought that on that very day Lord Kṛṣṇa had killed the demon Aghāsura and was now returning home with them.