Al-Kahf 18:2

Qayyiman liyunthira ba/san shadeedanmin ladunhu wayubashshira almu/mineena allatheenayaAAmaloona assalihati anna lahum ajran hasana

[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 [i.e., Paradise].

a straightforward Book to warn of a severe punishment from Him, and to give the good news to the believers, who do righteous deeds, that they will have an excellent reward (Paradise)