Al-Kāfi - Volume 1 > The Imams are those who have received Knowledge and it is firmly Established in their Hearts
Hadith #2

2ـ عَنْهُ عَنْ مُحَمَّدِ بْنِ عَلِيٍّ عَنِ ابْنِ مَحْبُوبٍ عَنْ عَبْدِ الْعَزِيزِ الْعَبْدِيِّ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ السَّلام) فِي قَوْلِ الله عَزَّ وَجَلَّ بَلْ هُوَ آياتٌ بَيِّناتٌ فِي صُدُورِ الَّذِينَ أُوتُوا الْعِلْمَ قَالَ هُمُ الائِمَّةُ (عَلَيْهم السَّلام )

2. It is narrated from him from Muhammad ibn Ali from ibn Mahbub from ‘Abd al-‘Aziz al-‘Abdi from abu ‘Abdallah (a.s.) who has said the following about the words of Allah, the Most Holy, the Most High. “In fact, the Quran consists of illustrious verses that exist in the hearts of those who have knowledge. . .” (29:49) ‘They are the Imams (a.s.)’.