SB 2.7.28
yad vai vraje vraja-pasun visatoya-pitan
palams tv ajivayad anugraha-drsti-vrstya
tac-chuddhaye ’ti-visa-virya-vilola-jihvam
uccatayisyad uragam viharan hradinyam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Then also when the cowherd boys and their animals drank the poisoned water of the river Yamuna, and after the Lord [in His childhood] revived them by His merciful glance, just to purify the water of the river Yamuna He jumped into it as if playing and chastised the venomous Kaliya snake, which was lurking there, its tongue emitting waves of poison. Who can perform such herculean tasks but the Supreme Lord?