Tafsir of Ar-Rum 30:35

"Or have We sent down to them an authority..."

"Authority" (Sultan): The proof (hujja). Its "speaking" is a metaphor, just as you would say, "His book speaks of such-and-such." This is among the things the Qur'an expresses in this manner. Its meaning is indication and testimony, as if it said: "It testifies to their polytheism and its validity."

"By what they were..." (bima kanu): The ma is a verbal noun (masdariyyah), meaning: "by their associating partners with Allah."

It is also permissible for it to be a relative pronoun (mawsulah), with the pronoun referring back to it. Its meaning would be: "It speaks of the matter which causes them to associate partners."

It is also possible that the meaning is: "Or have We sent down to them an authority," meaning: "an angel with a proof, and that angel speaks of the proof because of which they associate partners."

"And when We give the people a taste of mercy, they rejoice in it; but if an evil afflicts them for what their hands have put forth, behold, they despair."