Muhammad declared himself to be a mortal like unto any other (see 41:6) and therefore liable to make mistakes and fallible. When the prophet 1) withheld knowledge, 2) turned from the blind man, and 3) sought to teach the rich instead, he showed his fallibility (see 80:1-10). The quran is a guide to the path that is straightest (see 17:9). If every act of the prophet was divinely inspired why is it that God must admonish him several times in the quran to not stray (see 10:15, 17:74-74, 46:9, and 69:43-47)? Also in verses 10:15 and 46:9 as well as in verse 6:50 the prophet is commanded to follow the quran. Since we know the messenger is not infallible how do we obey the messenger of God? We obey the messenger by following God's commands and teachings for the messenger's duty was naught but to deliver the message plainly (see 16:35, 16:82, 29:18, and 36:16-17). Plain conveyance means conveying the message of the quran without alteration/change such as interpretations or explanations which are only to be God's part (see 75:16-19). Verse 3:79 says that the messengers would say to only teach and study the scripture, nothing more, just the scripture. Obeying the messenger/prophet means in essence obeying the quran. Take note also that without delivering the quran Muhammad is not delivering God's message.

O messenger, deliver what was sent down to you from your Lord, and if you do not then you have not delivered His message; and God will protect you from the people. God does not guide the disbelieving people. (5:67)

Here is a similitude that can be related to muslims who are
1) owned by the prophet (against his wishes of course), 2) scholars, 3) mullahs, 4) imams and 5) schools of thought and thus they are endlessly fighting/quarrelling.

God cites the example of a man who has partners that dispute with each other, and a man who has given to dealing with only one man. Are they the same? Praise be to God; most of them do not know.
(39:29)

It is made clear that having one Lord is best. Also those with partners at variance will be in a cloud of confusion regarding which is truly the correct way/deen. One Lord is all that is necessary, leave the others alone, and hold fast to the one Lord. God is the one lord/master held by man, the imams, mullahs, scholars, hadith/sunnah of Muhammad are the other lords/masters.

Another example would include extra-quranic innovation such as the hadith collections of muslim scholars, or the sunnah besides the ones mentioned by God. They are at variance with the quran, and push the idea of being authoritative. There is but one law (13:37) to God's deen, the quran alone. This situation can hold true with leaders of the people as well who are followed with barely a backward glance, i.e. inheritance (quran 2:170) instead of actual self-understanding of the clear precepts to be learned.
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