Narrations from Abu Bakr and Ali on "disjointed letters"

In "The Study Quran (Seyyed Hossein Nasr, Ed.)" and few other sources, I have come across these two narrations: > “Every book has a mystery (sirr), and the mystery of the Quran is the > beginnings of the sūrahs.” ʿAlī, the fourth Caliph, is reported to > have said, “Every book has a quintessence (ṣafwah), and the > quintessence of this Book is the spelled-out letters.” The source of these narrations are not given. Would anybody know the real source, and their reliability?