Tafsir of Fussilat 41:18

(And We saved) from that thunderbolt (those who believed and were fearing) by virtue of their faith and their persistence in piety. The intended meaning here is piety toward Allah (Mighty and Majestic is He). It has also been said that it means piety regarding the thunderbolt; however, [the interpretation that] one who fears the punishment of Allah Almighty is [thereby] a person of piety toward Allah (Glorified be He) is more sound, and that [the former interpretation] is not strong.