Surah Al-Kahf, Verse 2
Translation — Sahih International
[He has made it] straight, to warn of severe punishment from Him and to give good tidings to the believers who do righteous deeds that they will have a good reward
Source
Kur'an-ı Kerim, Kehf Suresi 18:2
https://kuran.diyanet.gov.tr/mushaf/kuran-meal-2/18-suresi-2-ayet