Quran 22:46

The Al Qur'an was revealed by Allaah, may He be glorified, as a law and a reference as to what is permissible and prohibited, what is commanded and forbidden, for people to follow, so that they obey its commands and abide by its prohibitions, take what it allows as permissible and take what it prohibits as forbidden. The Qur’an tells us about what happened before and what is yet to come, and it is a reference for judging between us.

Allaah says (interpretation of the meaning): “…
We have neglected nothing in the Book…”
[al-An’aam 6:38].

After the Revelation was completed, Allaah said (interpretation of the meaning):
“… This day, I have perfected your religion for you…”
[al-Maa’idah 5:3].

The Sunnah [words and deeds of the Prophet Muhammad(Peace and Blessings of Allaah be upon him)] came to explain and complement the Qur’an. The Prophet(Peace and Blessings of Allaah be upon him) said:
“I have been given the Qur’an and something like it with it.”
The phrase “something like it with it” refers to the Sunnah. (Saheeh hadeeth).

Allaah commanded us to refer to these two constitutional references, when He said (interpretation of the meaning):
“…if you differ in anything amongst yourselves, refer it to Allaah and His Messenger…”
[al-Nisa’ 4:59].
Referring to Allaah means referring to the Qur’aan, and referring to His Messenger Prophet Muhammad(صلى الله عليه و سلم) means referring to the Sunnah. The Qur’an is the primary source of legislation, then comes the Sunnah!