SB 2.7.28

yad vai vraje vraja-paśūn viṣatoya-pītān
 pālāṁs tv ajīvayad anugraha-dṛṣṭi-vṛṣṭyā
tac-chuddhaye ’ti-viṣa-vīrya-vilola-jihvam
 uccāṭayiṣyad uragaṁ viharan hradinyām
Synonyms: 
yat — one who; vai — certainly; vraje — at Vṛndāvana; vraja-paśūn — the animals thereof; viṣa-toya — poisoned water; pītān — those who drank; pālān — the cowherd men; tu — also; ajīvayat — brought to life; anugraha-dṛṣṭi — merciful glance; vṛṣṭyā — by the showers of; tat — that; śuddhaye — for purification; ati — exceedingly; viṣa-vīrya — highly potent poison; vilola — lurking; jihvam — one who has such a tongue; uccāṭayiṣyat — severely punished; uragam — unto the snake; viharan — taking it as a pleasure; hradinyām — in the river.
Translation: 
Then also when the cowherd boys and their animals drank the poisoned water of the river Yamunā, and after the Lord [in His childhood] revived them by His merciful glance, just to purify the water of the river Yamunā He jumped into it as if playing and chastised the venomous Kāliya snake, which was lurking there, its tongue emitting waves of poison. Who can perform such herculean tasks but the Supreme Lord?