The format of this will be many quranic verses. I will insert brackets in the places that Allah could have even mentioned the sunnah
Perfect places for Allah to mention the Sunnah:
I have shown the first verse:
Say (O Muhammad), "What is the greatest testimony?" Say, "God is witness between me and you that this Quran [And sunnah] has been inspired to me to warn you with it and whomever it reaches." 6:19
Many more to come:
[4:82] Then do they not reflect upon the Qur'an [and sunnah]? If it had been from [any] other than Allah , they would have found within it much contradiction
[12.2] Surely We have revealed it-- an Arabic Quran [and sunnah]-- that you may understand.
[12.3] We narrate to you the best of narratives, by Our revealing to you this Quran [and sunnah], though before this you were certainly one of those who did not know.
[17.82] And We reveal of the Quran [and sunnah] that which is a healing and a mercy to the believers, and it adds only to the perdition of the unjust.
[18.54] And certainly We have explained in this Quran [and sunnah] every kind of example, and man is most of all given to contention.
[20.2] We have not revealed the Quran [and sunnah] to you that you may be unsuccessful.
[20.113] And thus have We sent it down an Arabic Quran [and sunnah] , and have distinctly set forth therein of threats that they may guard (against evil) or that it may produce a reminder for them.
[20.114] Supremely exalted is therefore Allah, the King, the Truth, and do not make haste with the Quran [and sunnah] before its revelation is made complete to you and say: O my Lord ! increase me in knowledge.
25.30] And the Apostle cried out: O my Lord! surely my people have treated this Quran [and sunnah] as a forsaken thing.
[25.32] And those who disbelieve say: Why has not the Quran [and sunnah] been revealed to him all at once? Thus, that We may strengthen your heart by it and We have arranged it well in arranging.
[27.6] And most surely you are made to receive the Quran [and sunnah] from the Wise, the Knowing God.
I was planning on doing more verses as there are a ton more, but you get the idea. If Allah wanted to say that there was another source of law besides the Quran he would not have:
1.) Stated that we can only believe in the verses in the Quran
2.) Not even hint at this other source of law