Srimad Bhagavatam Canto 10, Chapter 14, Text 48

SB 10.14.48

adyanena maha-vyalo
yasoda-nanda-sununa
hato ’vita vayam casmad
iti bala vraje jaguh
 
Translation:
 
As the cowherd boys reached the village of Vraja, they sang, “Today Krsna saved us by killing a great serpent!” Some of the boys described Krsna as the son of Yasoda, and others as the son of Nanda Maharaja.
 
Purport: 
 
Actually, Lord Krsna had killed the demon Aghasura one year earlier, but the boys, bewildered by the mystic potency of Brahma for one year, did not notice the time passing and thus thought that on that very day Lord Krsna had killed the demon Aghasura and was now returning home with them.
 
Srimad Bhagavatam Canto 10, Chapter 14, Text 47
Srimad Bhagavatam Canto 10, Chapter 14, Text 49