Al-Kāfi - Volume 1 > Tacit and Explicit Testimony as proof of abu Muhammad al-Hassan ‘s (a.s.) Divine Authority over the people after Ali ibn Muhammad (a.s.)
Hadith #11

11ـ عَلِيُّ بْنُ مُحَمَّدٍ عَنْ إِسْحَاقَ بْنِ مُحَمَّدٍ عَنْ مُحَمَّدِ بْنِ يَحْيَى بْنِ دَرْيَابَ عَنْ أَبِي بَكْرٍ الْفَهْفَكِيِّ قَالَ كَتَبَ إِلَيَّ أَبُو الْحَسَنِ (عَلَيْهِ السَّلام) أَبُو مُحَمَّدٍ ابْنِي أَنْصَحُ آلِ مُحَمَّدٍ غَرِيزَةً وَأَوْثَقُهُمْ حُجَّةً وَهُوَ الاكْبَرُ مِنْ وَلَدَيَّ وَهُوَ الْخَلَفُ وَإِلَيْهِ يَنْتَهِي عُرَى الامَامَةِ وَأَحْكَامُهَا فَمَا كُنْتَ سَائِلِي فَسَلْهُ عَنْهُ فَعِنْدَهُ مَا يُحْتَاجُ إِلَيْهِ.

11. Ali ibn Muhammad has narrated from Ishaq ibn Muhammad from Muhammad ibn Yahya ibn Daryab from abu Bakr al-Fahfaki who has said the following. “He wrote to me as herein below. “My son, abu Muhammad instinctively is the most considerate in the family of Muhammad (s.a). He possesses the strongest authoritative supporting evidence above them and he is the eldest of my sons. He is the succeeding Imam and to him go the rings of Imamat (Leadership with Divine Authority) and its rules. Whatever you wanted to ask me ask it from him. With him is whatever he may need.”