Al-Baqarah 2:23

Wa-in kuntum fee raybin mimma nazzalnaAAala AAabdina fa/too bisooratin min mithlihi wadAAooshuhadaakum min dooni Allahi in kuntum sadiqeen

And if you are in doubt about what We have sent down [i.e., the Qur’ān] upon Our Servant [i.e., Prophet Muḥammad (ﷺ)], then produce a sūrah the like thereof and call upon your witnesses [i.e., supporters] other than Allāh, if you should be truthful.

If you are in doubt about what We have revealed to Our servant, then bring a Sūrah similar to this, and do call your supporters other than Allah, if you are true.