This is a common question posed by those who follow external sources besides the Quran, they ask this question to push their idea of following other sources besides the Quran. These sources are falsely attributed to the messenger. These sources are normally referenced as the hadith and sunnah.
Would I obey the messenger if I were alive when he was?
Yes, if the message he gave was the Quran as the Quran was revealed as the revelation to the messenger.
I want to stress when I say "yes", I am saying yes to the message he was delivering as opposed to what the prophet did in his personal life. For example, when the prophet says to you X, Y or Z was revealed to him, I would have obeyed him, BUT, I would do it if it was a Quranic verse and after pondering and reflecting over it (38:29)
[This is] a blessed Book which We have revealed to you, [O Muhammad], that they might reflect upon its verses and that those of understanding would be reminded. (38:29)
I would not, however, blindly follow his personal life or copy him for the sake of copying him. As a human being, he had his personal life just like everyone else and was prone to mistakes too.
And those who, when reminded of the verses of their Lord, do not fall upon them deaf and blind. (25:73)
Say (O Muhammad), "I am not a novelty among the messengers, nor do I know what will happen to me or to you. I follow nothing other than what is revealed to me (Quran). I am no more than a clear warner." 46:9
When Our clear revelations are recited to them, those who do not long for the meeting with Us say, "Bring a Quran other than this or replace it." Say, "It is not for me to replace it of my own accord. I only follow what is revealed to me. I fear if I disobey my Lord, the punishment of a great Day." 10:15
We are also instructed to adhere to and follow the Quran alone, ensuring we do not follow other sources besides it. The many verses below also highlight this fact:
These are God's revelations (Quran) that We recite to you with truth, so in which hadith other than God and His revelations (Quran) do they believe? 45:6
Which hadith after it (Quran) do they believe in? 7:185
So, in which hadith after it (Quran) do they believe? 77:50
A Book has been brought down to you, so let there be no constraint in your chest because of it, and so that you may warn with it. It is a Reminder for the believers.
Follow what has been brought down to you from your Lord and do not follow any allies besides Him. Rarely do you remember! 7:2-3
Or do you have some book in which you are studying? 68:37
And this is a blessed Book which We brought down, so follow it and be reverent so that you may attain mercy. 6:155
This (Quran) is not a fabricated hadith, but an authentication of what came before it, a detailed account of all things, and guidance and mercy for people who believe. 12:111
We did not leave anything out of the Book. 6:38
We brought the Book down to you providing explanations of all things. 16:89
The Quran tells us the messenger was forbidden from giving any instructions outside the Quran:
It (Quran) is a revelation from the Lord of the worlds. Had he (Muhammad) falsely attributed any sayings to Us, we would have grabbed him by the right, and We would have severed his aorta. None of you would be able to prevent it. And indeed, it is a reminder for the reverent. 69:43-48
What did the messenger follow?
Say, "What thing is the greatest testimony?" Say, "God is Witness between me and you, and this Quran has been revealed to me to warn you with it and whomever it reaches. Do you bear witness that there are other gods besides God?" Say, "I do not bear witness." Say, "He is but One God and I am innocent of the shirk that you commit." 6:19
We brought down to you the Book (Quran) with truth, confirming what came before it of the Scripture and superseding it. So, rule among them in accordance with what God has brought down, and do not follow their personal desires in place of what has come to you of the truth.5:48
To follow the messenger is to adhere to the revelation the messenger left with us, which is the Quranic message, not anything external to it!
To follow the messenger is to adhere to the message of the Quran as that is what the messenger followed and delivered.
Following something external to the Quran is following another god besides God.
The question now is, why are you following other sources besides the Quran when the Quran tells you not to?