Updated: Sep 6, 2020
The core of the faith is the "Shahada" (Testimony of Faith). This very first pillar of Islam is stated clearly in the Quran exactly as it was revealed to the recipients of previous Scriptures.
The Testimony of faith in the Torah (Old Testament) reads:
"Hear, O' Israel: The Lord our God, the Lord is One" Deuteronomy 6:4-5
The same Testimony is confirmed in the New Testament in the words of Jesus (Mark 12:29).
The Testimony authorised by God in the Quran affirms the same truth:
God bears witness that there is no god except Him, and so do the angels and those who possess knowledge. 3:18
The correct Testimony in all the Scriptures does not contain any name other than the name of God. The only Shahada (Testimony) authorised by God in the Quran is the one in 3:18.
The words "there is no god but He (God)", are the exact words which God Himself utters, also the angels and those who possess knowledge.
Most Muslims today are not satisfied with the Shahada given in 3:18 so they add to it the words: "and I bear witness that Muhammad is His messenger".
To add any words or names to a Testimony authorised by God is, in fact, to imply that the Testimony given to us by God in the Quran is incomplete! Or that God forgot the second half of the Testimony!
God is not forgetful nor does God make mistakes! We are also told that the Quran is fully detailed (6:114) and that God has not left anything out of the Book (6:38).
We are given in the Quran a great criterion for distinguishing the sincere believers from those who insist on idolising humans. In 39:45, we are told that those who do not believe will not be able to proclaim God's name on His own, they must include the name of their idol/s to be satisfied:
When God alone is mentioned, the hearts of those who do not believe in the Hereafter are filled with aversion, but when others are mentioned besides Him, they rejoice. 39:45
The Testimony is one such criterion where true believers are tested in the purity of their faith.
The true believers will have no problem uttering the correct Testimony which is given in the Quran (3:18) and which has no name other than the name of God. On the other hand, the ones who have set up idols besides God will never be satisfied with the Quranic Testimony on its own.
They will not be able to utter the Quranic Shahada without adding a further testimony for Muhammad.
The correct Testimony must contain the name of God and no one else is the fact that we are commanded in the Quran not to make any distinction between any of God's messengers:
The messenger has believed in what was brought down to him from his Lord, and so did the believers. They all believed in God, His Angels, His Scriptures and His Messengers: "We do not distinguish between any of His Messengers," and they said, "We hear and we obey. Forgive us, our Lord. To You is the ultimate destination." 2:285
Muhammad is indeed the messenger of God, but so are Abraham, Moses, Jesus, David and all the others. We should not make a special preference for Muhammad, otherwise, we violate the rule in 2:285. We are also told that Muhammad is only God's servant and he is not any different from any of the other messengers of God.
We are commanded clearly in the Quran to follow the Quran and nothing else. As a result, all those who utter the Shahada which includes the name of prophet Muhammad are violating the Quranic law.
Please verify everything you have read with the Quran.
Peace and Respect!