Meaning: There is no God but You, Glorified be You! Truly, I have been of the wrongdoers
This supplication was made by the Companion of the Fish (Prophet Yunus (as)) in the belly of the fish. If any Muslim supplicates in these words, his supplication will be accepted/responded to.
The Prophet (peace and blessings be upon him) said, “The supplication of my brother Dhun Nun (Yunus, peace be upon him), who called on Allah while in the whale’s belly: ‘There is no deity but You. Glory be to You! Verily, I have been among the wrongdoers’ (Quran 21:87) – no Muslim person says it, for any situation whatsoever, except that Allah Most High answers his call.” [Tirmidhi]
Surely Allah loves those who turn unto him in repentance and loves those who purify themselves.
Reference: Surat Al-Anbya Verse 87