God does not fear to describe a likeness for that which He has likened. (1)

As for the mā that accompanies mathalan, it has the meaning of alladhī (that which), because the meaning of the speech is: Indeed, God is not ashamed to set forth that which is a gnat in smallness and insignificance, or anything above it—as a parable.

If someone were to say to us: If the view regarding that is as you have said (2), then what is the reason for baʿūḍatan (a gnat) being in the accusative case, when you know that the interpretation of the words according to what you interpreted (3) is: "Indeed, God is not ashamed to set forth as a parable that which is a gnat"—so baʿūḍatu (a gnat) according to your statement should be in the nominative case? Whence, then, did the accusative case come to it?

It is said: The accusative case came to it from two aspects. One of them is that since mā was in the accusative position due to His saying "set forth", and baʿūḍa was its relative clause, it was inflected with its inflection (4) and bound to its grammatical case, just as Ḥassān ibn Thābit said:

And sufficient excellence for us over whoever else besides us ... is the love of the Prophet Muḥammad for us. (5)

Thus ghayr was inflected with the inflection of man. And the Arabs do that specifically with man and mā (6): they inflect their relative clauses with their inflection, because they are sometimes definite and sometimes indefinite.