The Misinterpretation of 3:86
Two words - "witnessed" and "messenger" - mined from a verse about disbelievers, to authorize a testimony God never decreed.
I. The Verse and the Claim
"Why should GOD guide people who disbelieved after believing, and after witnessing that the messenger is truth, and after solid proofs have been given to them? GOD does not guide the wicked." (3:86)
The Quran decrees exactly one proclamation of faith, and God Himself utters it first:
"GOD bears witness that there is no god except He, and so do the angels and those who possess knowledge. Truthfully and equitably, He is the absolute god; there is no god but He, the Almighty, Most Wise." (3:18)
One witness, one content: La ilaha illa Allah - there is no god except God. Those who append a second testimony naming Muhammad rarely engage 3:18; instead they reach for 3:86, arguing that since the verse contains "witnessed" and "the messenger," it authorizes bearing witness for the messenger. Two observations end the argument, and a third turns the verse around entirely.
II. Law Is Not Derived From What Disbelievers Did
First, look at whom 3:86 describes: people who disbelieved - who abandoned the faith after solid proofs. Every ritual and proclamation God decreed for believers arrives as a direct command: "you shall," "do not." No act of worship anywhere in the Quran is legislated by narrating the conduct of the wicked whom "GOD does not guide." Deriving a required testimony from this verse is deriving religion from a description of its deserters.
Second, the words claimed do not say what the claim needs. "Witnessing that the messenger is truth" means they had seen for themselves that the messenger was genuine - and rejected him anyway. That is a statement about the evidence they sinned against, not a liturgical formula to be recited. The Quran does record a spoken testimony about Muhammad's messengership - once - and the speakers are identified:
"When the hypocrites come to you they say, 'We bear witness that you are the messenger of GOD.' GOD knows that you are His messenger, and GOD bears witness that the hypocrites are liars." (63:1)
Rashad Khalifa's footnote on 63:1 presses the observation to its edge: this is "the only place in the Quran where such a statement is made. Only the hypocrites make such a statement." The proclamation was true in content - God confirms "you are His messenger" - yet God calls its utterers liars, because they professed with the tongue what the heart withheld, and because the decreed Shahadah (3:18) mentions no name but God's. Adding Muhammad to it - while omitting Abraham, Moses, Jesus, and the rest - also violates the command to make no distinction among His messengers (2:285).
III. What 3:86 Is Actually About
Read in its passage (3:82-90), the verse describes a specific tragedy: people who believed, who witnessed a truthful messenger, who received solid proofs - and then disbelieved. Rashad's footnote identifies the "solid proofs" as the Quran's mathematical code and reads the passage as a warning about rejecting God's Messenger of the Covenant: disbelieving after believing. Either way the direction of the verse is the same, and it is the opposite of the use it has been put to. 3:86 does not institute a testimony for a messenger; it condemns those who had every proof a messenger is sent with and turned wicked regardless. The verse the innovators quote for their second Shahadah is, in fact, a verse about the fate of those who abandon what God's proofs establish - while the only Shahadah God ever decreed stands two pages earlier, complete, with no name in it but His.