The matter is flexible; it is permissible to
say "Muhammad" or "Sayyiduna Muhammad" (peace be upon him) because he (peace be upon him) is the master of the former and the latter peoples. However, a Muslim should not add the epithet "Sayyiduna" to the Prophet's name in the Adhan (call to Prayer) and Iqamah (call to start the Prayer). Muslims should adhere to the formulas mentioned in the Hadith, saying: (Part No. 24; Page No. 150)
I bear witness that Muhammad is the Messenger of Allah.
The same applies to Tashahhud (a recitation in the sitting position in the second/ last unit of Prayer) in which the epithet "Sayyiduna" should not be added while reciting it. Adhering to the formulas on these occasions shows greater veneration of the Sunnah (whatever is reported from the Prophet).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.