ENOfficial
Sa'd reported God’s messenger as saying that no Muslim will supplicate God for anything and fail to receive an answer when he uses Dhun Nun’s supplication when he was in the belly of the fish: “There is no god but Thee to whom be the glory. I was indeed a wrongdoer.” [Qur'an, xxi, 87] Ahmad and Tirmidhi transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ سَعْدَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: دَعْوَةُ ذِي النُّونِ إِذا دَعَا رَبَّهُ وَهُوَ فِي بَطْنِ الْحُوتِ (لَا إِلَهَ إِلَّا أَنْتَ سُبْحَانَكَ إِنِّي كُنْتُ مِنَ الظالمينَ) لَمْ يَدْعُ بِهَا رَجُلٌ مُسْلِمٌ فِي شَيْءٍ إلاَّ استجابَ لَهُ ". رَوَاهُ أَحْمد وَالتِّرْمِذِيّ
sunnah.com