What says the Quran about sunnah?
The word sunnah (p. sunan) is used 16 times in the Quran. But this word is never attributed to Prophet Muhammad!
The Quran uses the word sunnah in the meaning of law and path (Al Imran  137; al-Anfal  38; al-Hijr  13; al-Isra  77; al-Kahf  55; an-Nisa  26). The term Sunnatullah (sunnah of Allah) is used for the unchangeable social laws for the communication between Allah and his servants.
“This is the sunnah/law of God which existed before, and you will never find any change in His sunnah.” (al-Fath  23. Also look at al-Ahzab  38; Fatir  43; al-Ghāfir  85. An example for the social laws: al-Ra’d  11). Sunnatullah is used in the Quran for the social laws, not for the laws of Nature:
“There can be no difficulty to the Nabi/Prophet in what Allah has indicated to him as a duty. It was the sunnah/law of Allah سُنَّةَ اللّٰهِٰ amongst those of old that have passed away. God’s order will be fulfilled.” (al-Ahzab  38).
Therefore, the Quran doesn’t tell anything about the traditional sunnah. The only sunnah in the Quran is the sunnah of Allah which is used for the unchangeable social laws.
Special thanks to Professor Abdulaziz Bayindir; Mustafa Islamoglu; Professor Mustafa Ozturk; Professor Ilyas Celebi, “Sunnetullah,” Encyclopedia of Islam (Diyanet, Turkey), XXXVIII, 159-160.