What is the ruling of Islamic law on the muezzin's saying "I bear witness that our master Muhammad is the Messenger of Allah" in the Adhan?

The first is not to add the word "Sayyidina" (our master) in the Adhan (call to prayer) and other prescribed ذكر (remembrances) and supplications, and the Tashahhud (testimony of faith); because the Prophet, peace and blessings be upon him, and his Companions and the Tabi'in (generation after the Companions) did not add it. Following the prescribed wordings is more preferable. Some scholars, and this is the relied-upon opinion among the Shafi'is and Hanafis, see it as desirable to add the word "Sayyidina" in the Adhan, the Tashahhud, and the prayers upon the Prophet, peace and blessings be upon him, because it is an honor and glorification for him, peace and blessings be upon him, and because adding a statement of reality is itself an act of good manners.