Allaah (The Most High) said:
يَا أَيُّهَا الَّذِينَ آمَنُوا لَا تُقَدِّمُوا بَيْنَ يَدَيِ اللَّهِ وَرَسُولِهِ ۖ وَاتَّقُوا اللَّهَ ۚ إِنَّ اللَّهَ سَمِيعٌ عَلِيمٌ
O you who believe! Do not put (yourselves) forward before Allah and His Messenger, and fear Allah. Verily! Allah is All-Hearing, All-Knowing. [49:1]
يَا أَيُّهَا الَّذِينَ آمَنُوا لَا تَرْفَعُوا أَصْوَاتَكُمْ فَوْقَ صَوْتِ النَّبِيِّ
O you who believe! Raise not your voices above the voice of the Prophet…[49:2]
Imaam Ibnul Qayyim (rahimahullaah) said:
When it is the case that Allaah forbade (them) from giving precedence to (themselves) over the Messenger, then which type of giving precedence to oneself over the Messenger (sallal-laahu-alayhi-wasallam) can be more severe than giving precedence to one’s intellect over the revelation he was sent with?
Many of the Salaf have stated (that this ayah means): ‘’Do not say anything (about the religion) until the messenger speaks and do not act until he commands you.’’
The most disobedient person amongst the people and the most severe with regards to giving precedence to him (or herself) over the Messenger is that one who gives precedence to his (or her) intellect or the intellect of another person over the revelation the Messenger was sent with. And when it is the case that Allaah has indeed forbidden them from raising their voices above the voice of the Messenger, then how about raising their intellects above the statements of the Messenger and the revelation he came with?
[Source: Al-Waabil as-Sayyib’ page: 21. Abridged and slightly paraphrased]