Is there a narration in which Imam Ali (a) was asked about his greatest Fadhail and he referred to verse 13:43, confirming his knowledge of the Book?
Yes. It is narrated in Al-Ehtijaaj by al-Tabarsi that Ameerul Momineen Ali (AS) was asked about best tribute for him, he replied by reciting the verse 13:43.
" الاحجتاج " للطبرسي سأل رجل عليّ بن أبي طالب (عليه السّلام) عن أفضل منقبة له ، فقرأ الآية ، وقال : « إيّانا عنى بمَن عنده علم الكتاب » .
Wassalam.