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 #13

13ـ الْحُسَيْنُ بْنُ مُحَمَّدٍ عَنِ الْخَيْرَانِيِّ عَنْ أَبِيهِ قَالَ كُنْتُ وَاقِفاً بَيْنَ يَدَيْ أَبِي الْحَسَنِ (عَلَيْهِ السَّلام) بِخُرَاسَانَ فَقَالَ لَهُ قَائِلٌ يَا سَيِّدِي إِنْ كَانَ كَوْنٌ فَإِلَى مَنْ قَالَ إِلَى أَبِي جَعْفَرٍ ابْنِي فَكَأَنَّ الْقَائِلَ اسْتَصْغَرَ سِنَّ ابي جعفر (عَلَيْهِ السَّلام) فَقَالَ أَبُو الْحَسَنِ (عَلَيْهِ السَّلام) إِنَّ الله تَبَارَكَ وَتَعَالَى بَعَثَ عِيسَى ابْنَ مَرْيَمَ رَسُولاً نَبِيّاً صَاحِبَ شَرِيعَةٍ مُبْتَدَأَةٍ فِي أَصْغَرَ مِنَ السِّنِّ الَّذِي فِيهِ أَبُو جَعْفَرٍ (عَلَيْهِ السَّلام )

13. Al-Husayn ibn Muhammad has narrated from al-Khayrani, from his father who has said the following. “Once I was standing before abu al-Hassan (a.s.) in Khurasan and someone said tp him, “O my master, if something will happen to who (will go Leadership with Divine Authority)?” The Imam (a.s.) said, “It will go to abu Ja‘far (a.s.), my son.” The person asking the question thought of abu Ja‘far (a.s.) as very young for such task. Abu al-Hassan (a.s.) said, “Allah, the Most Holy, the Most High, sent Jesus, son of Mary as a messenger prophet, the owner of a whole legal system. He began his task when he was smaller in age than abu Ja‘far (a.s.).”