Al-Allaamah Bin Ahmad an-Najmi ( May Allah have mercy on him ) said :

“As for the meaning of the declaration that Muhammad is the Messenger of Allah; it is the declaration that the Messenger is the Messenger of Allaah – obligated with the [task] of disseminating the Sharee’ah which he carried; his Ummah (nation) are obligated to believe in his message and that he does not legislate except that which Allaah has ordered him to legislate, and he did not utter except that which Allaah commanded him to convey.

Therefore, it is compulsory to obey him in what he ordered and to keep away from what he forbade. And [an act of] worship is not accepted from a servant except that which has been legislated by the Prophet. These  two declarations are the fundamental pillars of Islam and they are the conditions [to be fulfilled in order for] deeds [acts of worship] to be accepted. Deeds [i.e. acts of worship] of any servant will not be accepted except through belief in the Messengership of Prophet Muhammad and that it is obligatory to follow and obey him.”

At-Ta’leeqaat al-Bahiya ‘ala ar-Rasaa’il al-Aqeediyah / 131