لَّا إِلَهَ إِلَّا أَنتَ سُبْحَانَكَ إِنِّي كُنتُ مِنَ الظَّالِمِينَ
"There is no god except You; exalted are You. Indeed, I have been of the wrongdoers."
This dua is mentioned in Surah Ambia, Ayah No. 87.
This is the supplication of Prophet Yunus (as). When his people did not obey him after he invited them to the right way, then he left his city for somewhere else while Allah Almighty had given him no order about it, so Allah did not like prophet Yunus (as)’s decision. Consequently, he had to face a difficult trial in a way that when he was traveling by the boat, he was compelled to get down into the sea. There a big fish had swallowed him. He began to pray humbly to Allah with this supplication in the stomach of the fish. Finally, Allah (swt) granted his supplication and the fish had thrown him out on the bank of the sea.
One should read this dua at the time of trouble and difficulty.