The Manipulation of 4:80
"Whoever obeys the messenger is obeying God" - because the messenger carried nothing of his own to obey.
I. The Verse
"Whoever obeys the messenger is obeying GOD. As for those who turn away, we did not send you as their guardian." (4:80)
Hadith advocates read the first sentence as proof of a second body of law: if obeying the messenger equals obeying God, the messenger must have issued commands of his own, alongside God's - and those commands, they say, are the hadith. The verse is actually explaining the opposite, and three verses of the messenger's job description show how.
II. Why the Equation Holds
"The sole duty of the messenger is to deliver the message, and GOD knows everything you declare and everything you conceal." (5:99)
"Say, 'I am not different from other messengers. I have no idea what will happen to me or to you. I only follow what is revealed to me. I am no more than a profound warner.'" (46:9)
"...You shall rule among them in accordance with GOD's revelations, and do not follow their wishes if they differ from the truth that came to you..." (5:48)
The messenger's sole duty is delivery; his own conduct is bound to the revelation; even his rulings as judge are commanded to follow what God sent down. A messenger so constrained has exactly one voice with which to command anyone - God's message. That is why obeying him is obeying God: the equation of 4:80 holds because the two obediences have identical content, the Quran. The verse is a statement about the purity of the channel, not the existence of a second channel.
Run the manipulated reading in reverse and it destroys itself. If "obey the messenger" referred to personal teachings beside the revelation, then obeying the messenger would be obeying other than what God sent down - and 4:80 could never equate the two. The equation is only possible if the messenger adds nothing. God sealed the point in the same verse: "we did not send you as their guardian" - even enforcement was withheld from him. Delivery, warning, judgment by the Book; nothing else was his office, so nothing else is our obedience. Whoever obeys the messenger today opens the message he delivered - and obeys it.