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 #4

4ـ وَعَنْهُ عَنْ مُوسَى بْنِ جَعْفَرِ بْنِ وَهْبٍ عَنْ عَلِيِّ بْنِ جَعْفَرٍ قَالَ كُنْتُ حَاضِراً أَبَا الْحَسَنِ (عَلَيْهِ السَّلام) لَمَّا تُوُفِّيَ ابْنُهُ مُحَمَّدٌ فَقَالَ لِلْحَسَنِ يَا بُنَيَّ أَحْدِثْ لله شُكْراً فَقَدْ أَحْدَثَ فِيكَ أَمْراً.

4. From him who has narrated from Musa ibn Ja‘far ibn Wahab from Ali ibn Ja‘far who has said the following. “I was present with abu al-Hassan (a.s.) when his son Muhammad died. The Imam (a.s.) said this to (his son) al-Hassan (a.s.), “Son renew your thanks to Allah because He has just granted you the matter (Leadership with Divine Authority).”