Why “Muhammadun” in the kalima but “Muhammadan” in the adhan

Question

Why is it “Muhammadun rasulu-llah” (مُحَمَّدٌ رَسُولُ اللهِ) in the kalima but “Muhammadan rasulu-llah” (مُحَمَّدًا رَسُولُ اللهِ) in the adhan? And why is it wrong if one says [the word] with damma in the adhan and with fatha in the kalima?

Answer

This is precisely what the rules of the Arabic language require; reading contrary to them is wrong.

Source: Fatawa Mahmudiyya

Preview. The full text opens with JavaScript enabled.