SB 8.24.1
sri-rajovaca
bhagavañ chrotum icchami
harer adbhuta-karmanah
avatara-katham adyam
maya-matsya-vidambanam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maharaja Pariksit said: The Supreme Personality of Godhead, Hari, is eternally situated in His transcendental position, yet He descends to this material world and manifests Himself in various incarnations. His first incarnation was that of a great fish. O most powerful Sukadeva Gosvami, I wish to hear from you the pastimes of that fish incarnation.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Supreme Personality of Godhead is all-powerful, yet He accepted the form of an uncommon fish. This is one of the ten original incarnations of the Lord.