Is the "Raa" in "Allahu Akbar" pronounced with an "a" vowel (fatḥa) or an "u" vowel (ḍamma) when connected to the Basmala in the Adhan?

The muezzin should pronounce the Adhan slowly and distinctly, and he should combine every two takbirs with a sound. The most eloquent pronunciation is to vocalize the "r" of the first word "Akbar" with a dammah (u-sound) when connecting it [to the previous word "Allah"], because it is the predicate of the subject "Allah." Scholars have differed regarding the ruling on the ra sound; some said it should have a dammah when connecting, while others said it should have a fathah (a-sound), but the preponderant opinion is that it should have a dammah.