CC Ādi 5.78

yadyapi kahiye tāṅre kṛṣṇera ‘kalā’ kari
matsya-kūrmādy-avatārera tiṅho avatārī
Word for word: 
yadyapi — although; kahiye — I say; tāṅre — to Him; kṛṣṇera — of Lord Kṛṣṇa; kalā — part of the part; kari — making; matsya — the fish incarnation; kūrma-ādi — the tortoise incarnation and others; avatārera — of all these incarnations; tiṅho — He; avatārī — the original source.
Translation: 
Although Kāraṇodaśāyī Viṣṇu is called a kalā of Lord Kṛṣṇa, He is the source of Matsya, Kūrma and the other incarnations.