Al-Kāfi - Volume 1 > The Imams are those who have received Knowledge and it is firmly Established in their Hearts
Hadith #3

3ـ وَعَنْهُ عَنْ مُحَمَّدِ بْنِ عَلِيٍّ عَنْ عُثْمَانَ بْنِ عِيسَى عَنْ سَمَاعَةَ عَنْ أَبِي بَصِيرٍ قَالَ قَالَ أَبُو جَعْفَرٍ (عَلَيْهِ السَّلام) فِي هَذِهِ الايَةِ بَلْ هُوَ آياتٌ بَيِّناتٌ فِي صُدُورِ الَّذِينَ أُوتُوا الْعِلْمَ ثُمَّ قَالَ أَمَا وَالله يَا أَبَا مُحَمَّدٍ مَا قَالَ بَيْنَ دَفَّتَيِ الْمُصْحَفِ قُلْتُ مَنْ هُمْ جُعِلْتُ فِدَاكَ قَالَ مَنْ عَسَى أَنْ يَكُونُوا غَيْرَنَا.

3. It is narrated from him from Muhammad ibn Ali from ’Uthman ibn ‘Isa from Sama‘a from abu Basir from abu Ja‘far (a.s.) who has said the following about this verse of the Holy Quran. “In fact, the Quran consists of illustrious verses that exist in the hearts of those who have knowledge. . .” (29:49) The Imams (a.s.) then said, “O abu Muhammad, for the sake of Allah, be the judge and tell, of who it is spoken (so often) between the two covers of the Holy Quran?” I then asked, “Who are they, “May Allah take my soul in service for your cause. The Imams (a.s.) said, “Who else can they be other than ourselves?”