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 #1

1ـ عَلِيُّ بْنُ مُحَمَّدٍ عَنْ مُحَمَّدِ بْنِ أَحْمَدَ النَّهْدِيِّ عَنْ يَحْيَى بْنِ يَسَارٍ الْقَنْبَرِيِّ قَالَ أَوْصَى أَبُو الْحَسَنِ (عَلَيْهِ السَّلام) إِلَى ابْنِهِ الْحَسَنِ قَبْلَ مُضِيِّهِ بِأَرْبَعَةِ أَشْهُرٍ وَأَشْهَدَنِي عَلَى ذَلِكَ وَجَمَاعَةً مِنَ الْمَوَالِي.

1. Ali ibn Muhammad has narrated from Muhammad ibn Ahmad al-Nahdi from Yahya ibn Yasar al-Qanbar who has said the following. “Abu al-Hassan (a.s.) prepared his directive will to his son, al-Hassan four months before his leaving this world. He appointed me to bear testimony to his will as well as a group of the follower friends.”