CC Ādi 4.13

ataeva viṣṇu takhana kṛṣṇera śarīre
viṣṇu-dvāre kare kṛṣṇa asura-saṁhāre
Synonyms: 
ataeva — therefore; viṣṇu — Lord Viṣṇu; takhana — at that time; kṛṣṇera — of Lord Kṛṣṇa; śarīre — in the body; viṣṇu-dvāre — by Lord Viṣṇu; kare — does; kṛṣṇa — Lord Kṛṣṇa; asura-saṁhāre — killing the demons.
Translation: 
At that time, therefore, Lord Viṣṇu is present in the body of Lord Kṛṣṇa, and Lord Kṛṣṇa kills the demons through Him.