Al-'Ankabut 29:48

Wama kunta tatloo min qablihi min kitabinwala takhuttuhu biyameenika ithan lartabaalmubtiloon

And you did not recite before it any scripture, nor did you inscribe one with your right hand. Then [i.e., otherwise] the falsifiers would have had [cause for] doubt.

You (O Prophet,) have never been reciting any book before this, nor have you been writing it with your right hand; had it been so, the adherents of falsehood would have raised doubts.