Quran 15:50 from Surah Al-Hijr on Divine Marvels.
وَأَنَّ عَذَابِي هُوَ ٱلۡعَذَابُ ٱلۡأَلِيمُ
English translation: And that My punishment it (is) the punishment the most painful
Transliteration: wa-anna adhabi huwa al-'adhabu al-alimu