In Shafi’i fiqh, the sunnah method of saying "Allahu Akbar" in the adhan is to place sukoon (no vowel) on the Ra. This is the most preferred way. However, if someone says the takbeers together without a pause between them, then it is better to use pesh (u), saying "Allahu Akbaru." Even if someone uses zabar (a) or pesh (u), it is still allowed. But between zabar and pesh, using pesh is better than zabar.

This fatwa is based on trusted Shafi’i books like I’anat al-Talibin, Nihayat al-Muhtaj, and Hashiyat al-Jamal, where Islamic scholars have explained the proper way clearly.