Is it permissible to say to our Prophet: O Muhammad?

It is not permissible to address the Prophet, peace and blessings be upon him, with "O Muhammad" during his lifetime, out of reverence for him. Rather, one should say: "O Messenger of Allah," "O Prophet of Allah." Nor is it permissible to supplicate to him after his death, because supplication (du'a) is an act of worship due to Allah alone. Whoever supplicates to other than Allah has committed polytheism (shirk). Therefore, calling out "O Muhammad" or "O Messenger of Allah" with the intention of supplication and seeking something is major polytheism. However, if the intention is not supplication but merely remembrance, there is no harm in it, though saying "O Muhammad" is contrary to proper etiquette.