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?
The two hadiths that you quoted are related to Al-Hurūf al-Muqatta'ah. These are combinations of 1-5 alphabets that occur at the start of 29 chapters of the Qur'an which are recited as separate characters rather than syllables.
The two hadiths were quoted in some of the major classical tafsīrs of the Qur'an (e.g., Tafsir al-Baghawi and Tafsir al-Kabir, among others). However, all such quotes provided no associated narration chains (e.g., Tafsir al-Kabir, in Arabic only). Therefore, we have no means of authenticating the content of these two specific hadiths.
One another note, there are other hadiths similar to the ones above attributed to major companions, too, like 'Abdullah ibn Mas'ūd, Hasan of Basra, among others. All such hadiths are graded as mawqūf, i.e., the hadith is attributed to a companion, not to the Prophet ﷺ. All these hadiths endorse the same concept: there is no verse or hadith attributed to the Prophet ﷺ that explains the meaning of these letters; hence, whatever one reads about them is merely the personal opinion of the author.
