The second aspect is: We may concede to those who deny the Sunnah that the meaning of Dhikr is the Glorious Qur'an, and the pronoun in Allah's Saying: "to it" refers to the Qur'an which is called Dhikr too. However, the restriction by which they hold as a proof that the Sunnah did not enter the circle of conservation because the Qur'an only is preserved, and your claim that the Sunnah is invalid to be taken as evidence, and it is not a source of legislation; this restriction is not real but it is alleged.
The proof to that is the following:
1- Allah (Glory be to Him) preserves the heavens and the earth from destruction. Allah (may He be Exalted) says: "Verily, Allâh grasps the heavens and the earth lest they should move away from their places, and if they were to move away from their places, there is not one that could grasp them after Him. Truly, He is Ever Most Forbearing, Oft-Forgiving."(1)
Allah (Glory be to Him) says: "And He feels no fatigue in guarding and preserving them. And He is the Most High, the Most Great."(2)
2- Allah (Glory be to Him) protected His Prophet Muhammad (peace be upon him) from being killed. Allah (Glory be to Him) says: "Allâh will protect you from mankind."(3)
If the reality of restriction is nullified, the things resulted from will be nullified too which is: not recognizing the Sunnah as a source of legislation. If the restriction is alleged, it is permissible that Allah (Glorified and Exalted be He) has promised to preserve the Sunnah of the Prophet (peace be upon him) tacitly because the alleged restriction is meant for exaggeration along with including other things.
So, the promise of Allah of protecting the Sunnah is included in preserving the Qur'an because the Sunnah is an illustration and explanation to the Glorious Qur'an. Allah (Glory be to Him) says: "And We have also sent down to you (O Muhammad peace be upon him) the Dhikr [reminder and the advice (i.e. the Qur'ân)], that you may explain clearly to men what is sent down to them." (4)
1- [Surat Fatir: 41].
2- [Surat Al-Baqarah: 255].
3- [Surat Al Ma'idah: 67].
4- [Surat An-Nahl: 44].