Allah'S Protection Of Quran
The promise of God is for the protection of Revelation only (15:9). No such promise extends to any other source of religion including Sunnah. Therefore, all other sources are subject to critical scrutiny and investigation for their accuracy, and must be compliant with Quran and its clear verses, because Quran is Criterion for judging every other content. If anything is found non-compliant with Quran, it cannot be accepted as a legitimate source of religion.