Al-Kāfi - Volume 1 > Tacit and Explicit Testimony as proof of abu Ja‘far al-Thani’s (the second) (a.s.) Divine Authority over the people after abu al-Hassan al-Rida (a.s.)
Hadith #8

8ـ أَحْمَدُ عَنْ مُحَمَّدِ بْنِ عَلِيٍّ عَنِ الْحَسَنِ بْنِ الْجَهْمِ قَالَ كُنْتُ مَعَ أَبِي الْحَسَنِ (عَلَيْهِ السَّلام) جَالِساً فَدَعَا بِابْنِهِ وَهُوَ صَغِيرٌ فَأَجْلَسَهُ فِي حَجْرِي فَقَالَ لِي جَرِّدْهُ وَانْزِعْ قَمِيصَهُ فَنَزَعْتُهُ فَقَالَ لِيَ انْظُرْ بَيْنَ كَتِفَيْهِ فَنَظَرْتُ فَإِذَا فِي أَحَدِ كَتِفَيْهِ شَبِيهٌ بِالْخَاتَمِ دَاخِلٌ فِي اللَّحْمِ ثُمَّ قَالَ أَ تَرَى هَذَا كَانَ مِثْلُهُ فِي هَذَا الْمَوْضِعِ مِنْ أَبِي (عَلَيْهِ السَّلام )

8. Ahmad has narrated from Muhammad ibn Ali From al-Hassan ibn al-Jahm who has said the following. “Once I was in the presence of abu al-Hassan (a.s.). He called his son who was a small (boy). The Imams (a.s.) placed him in my lap and said to me. “ Move his shirt aside.” When I did so. The Imams (a.s.) said, “Look in between his shoulders.” I look and I found in the skin of one shoulder something like an imprint of a seal.” The Imams (a.s.) then asked, “Do you see this.? Similar to this there was one in the shoulder of my father.”