What is the authenticity of the hadith: "The Quran was revealed in seven ahruf (modes/letters), there is not a single حرف (harf) among them but it has an outer and an inner meaning, and indeed Ali ibn Abi Talib has knowledge of its outer and inner meanings"?

It is established that the Qur’an was revealed in seven ahruf (modes of recitation), and the Uthmanic Mus'haf compiled one of these ahruf. The seven ahruf differed in wording while maintaining unity in meaning. The hadith "Every ayah has a ẓahr (outward meaning) and a baṭn (inward meaning)" has been reported, but with weakness in its chain of transmission.

The ẓahr of an ayah refers to what is apparent of its meaning to the general public, while its baṭn refers to what is hidden of its meaning, known only to those with knowledge through deduction and jurisprudence.

As for the statement that Ali ibn Abi Talib possessed knowledge of both the ẓahr and the baṭn, this is not authentically reported from Ibn Mas'ud, and its chain of transmission is weak.