The Quran

وَذَا النّونِ إِذ ذَهَبَ مُغاضِبًا فَظَنَّ أَن لَن نَقدِرَ عَلَيهِ فَنادىٰ فِي الظُّلُماتِ أَن لا إِلٰهَ إِلّا أَنتَ سُبحانَكَ إِنّي كُنتُ مِنَ الظّالِمينَ

Muhammad Asad

AND [remember] him of the great fish¹ - when he went off in wrath, thinking that We had no power over him!² But then he cried out in the deep darkness [of his distress]: "There is no deity save Thee! Limitless art Thou in Thy glory! Verily, I have done wrong!"³

I.e., the Prophet Jonah, who is said to have been swallowed by a "great fish", as mentioned in 37:139 ff. and more fully narrated in the Old Testament (The Book of Jonah).
According to the Biblical account (which more or less agrees with the Qur'anic references to his story), Jonah was a prophet sent to the people of Nineveh, the capital of Assyria. At first his preaching was disregarded by his people, and he left them in anger, thus abandoning the mission entrusted to him by God; in the words of the Qur'an (37:140), "he fled like a runaway slave". The allegory of his temporary punishment and his subsequent rescue and redemption is referred to elsewhere in the Qur'an (i.e., in 37: 139-148) and explained in the corresponding notes. It is to that punishment, repentance and salvation that the present and the next verse allude. (The redemption of Jonah's people is mentioned in 10:98 and 37:147-148.)
Lit., "I was among the wrongdoers".