SB 10.16.6

taṁ caṇḍa-vega-viṣa-vīryam avekṣya tena
 duṣṭāṁ nadīṁ ca khala-saṁyamanāvatāraḥ
kṛṣṇaḥ kadambam adhiruhya tato ’ti-tuṅgam
 āsphoṭya gāḍha-raśano nyapatad viṣode
Word for word: 
tam — him, Kāliya; caṇḍa-vega — of fearsome power; viṣa — the poison; vīryam — whose strength; avekṣya — seeing; tena — by him; duṣṭām — contaminated; nadīm — the river; ca — and; khala — the envious demons; saṁyamana — for subduing; avatāraḥ — whose descent from the spiritual world; kṛṣṇaḥ — Lord Kṛṣṇa; kadambam — a kadamba tree; adhiruhya — climbing up on; tataḥ — from it; ati-tuṅgam — very high; āsphoṭya — slapping His arms; gāḍha-raśanaḥ — tying His belt firmly; nyapatat — He jumped; viṣa-ude — into the poisoned water.
Translation: 
Lord Kṛṣṇa saw how the Kāliya serpent had polluted the Yamunā River with his terribly powerful poison. Since Kṛṣṇa had descended from the spiritual world specifically to subdue envious demons, the Lord immediately climbed to the top of a very high kadamba tree and prepared Himself for battle. He tightened His belt, slapped His arms and then jumped into the poisonous water.
Purport: 

According to the ācāryas, Lord Kṛṣṇa also tied back the locks of His hair as He prepared to do battle with Kāliya.