Tafsir of Al-Qamar 54:23

(Thamud denied the warners.)

This refers to the messengers—peace be upon them—for denying one of them, namely Salih (peace be upon him) in this context, is a denial of all of them, due to their agreement upon the foundations of the religious laws. It is also possible that [the word nudhur] is an infinitive (verbal noun) or a plural. Furthermore, it is possible that it is a plural of nadhir, meaning that which one is warned against. Therefore, do not be heedless.