Question: A friend of mine says, “In order to have iman, it is not necessary to say the phrase ‘Muhammadun Rasul-Allah’ in the Kalima-i shahadat. Saying only ‘La ilaha ill-Allah’ will suffice.” Is the phrase “Muhammadun Rasul-Allah” not included in the Kalima-i shahadat, which is the first of the five fundamentals of Islam? Do we not say “Ash-hadu an la ilaha ill-Allah wa ash-hadu anna Muhammadan ’abduhu wa rasuluh” when we are reciting the Amantu? Is such an iman valid? What is the meaning of Kalima-i shahadat?
ANSWER
The meaning of Kalima-i shahadat is as follows:
(I bear witness, that is, I know and confirm by word of mouth as if I saw, that there is no ilah save Allah, and again I bear witness that Muhammad “alaihis-salam” is His human slave and Messenger.)
Belief in prophets is the fundamental principle of iman. A person who disbelieves in even one of the prophets, e.g., a person who says “Hadrat Adam was not a prophet,” is not a Muslim.

Excluding the part “Muhammadun Rasul-Allah” is a roundabout way of saying “Do not believe in the rules Islam has communicated,” because such a thought comes to mean ascribing ignorance to Allah (never!). If it were unnecessary, would it have been stated again and again in the Kalima-i shahadat, (the prayer) Attahiyyatu, adhan, ayat-i karimas, hadith-i qudsis.

Yet another meaning of believing is liking. If one dislikes a rule of Islam though one believes it, this causes disbelief. For example, if one believes the fact that it is sunnat to grow a beard but does not like it, one becomes a renegade. The situation being so, we omit to mention the credal state of a person who disapproves of the Prophet of the religion. 
About this matter, the Qur’an al-karim says (what means):

(We have sent you as a Mercy to all classes of beings.) [Surat-ul-Anbiya’ 107]

(Say: “O people! I am His Messenger that Allah has sent to you all.”) [Surat-ul-A’raf 158]

(We have sent you to all people as a bearer of good tidings and as a warner, but most of them do not know it.) [Surat-u Saba 28]

It is necessary to obey the Messenger
Does excluding the utterance of the second part of the Kalima-i shahadat come to mean that it is not necessary to obey the Messenger of Allah? However, Allahu ta’ala repeatedly orders us to believe in and follow Hadrat Muhammad (‘alaihis-salam) and warns that those who do not follow him cannot be Muslims; they are disbelievers. Some verses from the Qur’an say (what means):

(Obey My Messenger! So, you will be guided to the right way.) [Surat-un-Nur 54]

(Say: “Obey Allah and His Messenger!” If [if they do not obey] they pay no heed, [they become disbelievers and] surely Allah does not love the disbelievers.) [Surat-u Al-i ‘Imran 32]

(When Allah and His Messenger decreed a matter, from then on, a believing man and woman do not have the right to have personal preferences or options as to that matter.) [Surat-ul-Ahzab 36]