As for the meaning of the testification that Muhammad (صلى الله عليه و سلم) is the Messenger of Allaah [Muhammad Rasoolullaah] then it is the attestation with the tongue and the belief with the heart that Muhammad bin ‘Abdullaah al- Qurashee al-Haashimee is the Messenger of Allaah to all of the creation, to the Jinn and mankind, as stated by Allaah, The Most High:

Verily! I am Allaah’s Messenger to you all. He is the One to whom the Dominion of the heavens and the earth belongs, none has the right to be worshipped but Him; It is He Who gives life and causes death. So believe in Allaah and His Messenger, the Prophet who can neither read nor right, who believes in Allaah and in His Words [this Qur’aan] and follow him so that you may be guided. [Qur’aan, soorat al-A’raaf (7): 158].

He, The Most High, also Said:

(Exalted is) Blessed be He Who sent down the Criterion [of right and wrong, i.e. the Qur’aan] to His slave [Muhammad صلى الله عليه و سلم] that he may be a warner to the ‘Aalameen (mankind and Jinn). [Qur’aan, soorat al-Furqaan (25): 1].

This Shahaadah necessitates that

you believe whatever Allaah’s Messenger informed you of;you comply with whatever he commands;you avoid whatever he forbade and prohibited, and thatyou do not worship Allaah except with that which he prescribed.

It also necessitates that

you do not believe that Allaah’s Messenger has any share nor any right in Ar-Ruboobiyyah or in controlling the affairs of the creation, nor any right to be worshipped at all.Rather, he صلى الله عليه و سلم is a slave worshipper, not one to be worshipped; and that he is a Messenger who is not to be belied. It is not in his power to bring harm or benefit, neither for himself nor for others, except as Allaah Wills.

This is evident in the Saying of Allaah, The Most High:

Say [O Muhammad صلى الله عليه و سلم]: “I do not tell you that with me are the treasures of Allaah, nor that I know of al-Ghayeb (the hidden and unseen); nor I tell you that I am an angel. I but follow what is revealed to me (by inspiration).”[Qur’aan, soorat al-An’aam (6): 50].

He is a slave of Allaah who acts as he is commanded and follows the orders he is given. And Allaah, The Most High, Says:

[Say O Muhammad صلى الله عليه و سلم]: “It is not in my power to cause (bring) you harm or bring (guide) you to the Right Path.” Say: “None can protect me from Allaah’s Punishment [if I were to disobey Him], nor should I find refuge except in Him.” [Qur’aan, soorat al-Jinn (72): 21-22].

Allaah, The Most High, also Says:

Say (O Muhammad صلى الله عليه و سلم): “I possess no power of benefit or hurt to myself except as Allaah Will. And if I had the knowledge of the Ghayeb, I shoud have secured for myself an abundance of wealth, and no evil should have touched me. I am but a warner, and a bringer of glad tidings unto people who believe.” [Qur’aan, soorat al-A’raaf (7): 188].

So, this is the meaning of the testimony of Laa Ilaaha Illalaah, Muhammad Rasoolullaah. From this meaning the person knows that nothing from the creation deserves the right to be worshipped, not the Messenger of Allaah صلى الله عليه و سلم nor anyone from the creation who is below him in rank; and that worship an only be for Allaah, alone; and that the right of Allaah’s Messenger upon us is to give him the rank and standing which Allaah, The Most High, gave to him: The slave of Allaah and His Messenger.

Translated by Dr Saleh as Saleh rahimahullaah
Source : Understanding Worship – Fiqh ul-‘Ibadah – Shaykh ibn Uthaymeen

JazakhumLlahukhayra