The Quran makes this claim:
Verily, we have sent down the Reminder, and, verily, we will guard it. Q15:9
According to Muslim commentators like Ibn Kathir, this Reminder is the Quran since just a few verses before that we read in the same Surah:
But they say, ‘O thou to whom the Reminder has been sent down! verily, thou art possessed.’ Q15:6
