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 #6

6ـ عَلِيُّ بْنُ مُحَمَّدٍ عَنْ مُحَمَّدِ بْنِ أَحْمَدَ الْقَلانِسِيِّ عَنْ عَلِيِّ بْنِ الْحُسَيْنِ بْنِ عَمْرٍو عَنْ عَلِيِّ بْنِ مَهْزِيَارَ قَالَ قُلْتُ لابِي الْحَسَنِ (عَلَيْهِ السَّلام) إِنْ كَانَ كَوْنٌ وَأَعُوذُ بِالله فَإِلَى مَنْ قَالَ عَهْدِي إِلَى الاكْبَرِ مِنْ وَلَدَيَّ.

6. Ali ibn Muhammad has narrated from Muhammad ibn Ahmad al-Qalanisi from Ali ibn al-Husayn ibn ‘Amr from Ali ibn Mahziyar who has said the following. “Once I said abu al-Hassan (a.s.), “If something will happen-I seek refuge before Allah – to who then will go (Leadership with Divine Authority)?” He said, “My covenant (directive will) is for the eldest of my sons.”