Tafsir of Al-Mu'minoon 23:109

(Indeed, it was a party...)

The particle inna (indeed) serves as a causative explanation for what preceded it regarding the prohibition against [their] supplication. That is to say: the matter is—and it was recited by Ubayy and Harun al-'Ataki as annahu with a fathah on the alif—because the matter was that there was in the world, desiring a return to it, a party of My servants.

They are the believers; it is also said they are the Companions; and it is said they are the People of the Suffah—may Allah, the Exalted, be pleased with them all—(who used to say: "Our Lord, we have believed, so forgive us and have mercy upon us, and You are the best of the merciful.")