What Does the Quran Say About Sunnah?
Sunnah is mentioned 16 times in the Quran, but it is not used even once in reference to the Messenger of Allah.
Sunnah means the way followed (Lisân al-ʿArab, S-N-N art.). The phrase sunnatullah in the Quran means the laws that Allah put in order to create and maintain nature and to regulate social life:
“Allah’s sunnah / law (sünnetillah سُنَّةِ اللّٰهِ ) has always been like this from past to present. You will never find any change in the Sunnah of Allah.”
(al-Fath  23. See also al-Ahzab  38; Al Imran  137; al-Anfal  38; Fatir  43; al-Hidjr  13; al-Isra  77; al-Kahf  55; al-Ghafir/Mu’min  85; an-Nisa  26).
Therefore, the Quran does not mention the sunnah in the traditional understanding that emerged later. The only sunnah mentioned in the Quran is the sunnah of Allah, which expresses his unchanging practices and laws.
Note: According to the Quran, the only source that believers must follow is the Quran because Islam is complete (Maide  3). Believers will be questioned only about the Quran in the hereafter (al-Zukhruf  44).