It is to have firm affirmation from the bottom of one’s heart, together in conformity with speech upon the tongue, that Muhammad is His slave and Messenger to all of mankind and the jinn, “…as witness, and a bearer of glad tidings, and a warner. And as one who invites to Allaah by His Leave, and as a lamp spreading light.” (al-Ahzaab: 45,46)
So it is obligatory to affirm all that he mentioned regarding the past and future, all that he declared lawful or prohibited, with compliance and obedience to that which he ordered, and refraining and abstaining from all that he prohibited. Following his Sharee’ah, and clinging to his Sunnah in private and in public accompanied with pleasure with what he has decreed and submission to it. And knowing that obedience to him is in fact obedience to Allaah, and disobedience to him is disobedience to Allaah, as he is the Messenger sent by Allaah to convey His Message.
Allaah did not take his life until He perfected the deen through him. And he (sallallaahualaihi wasallam) conveyed the Message in a most perfect and complete way, and that he left his nation upon a clear white plain, no one deviates from it after him, except that he is destroyed. This topic will be discussed later, if Allaah wills.