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 #2

2ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ مُعَمَّرِ بْنِ خَلادٍ قَالَ سَمِعْتُ الرِّضَا (عَلَيْهِ السَّلام) وَذَكَرَ شَيْئاً فَقَالَ مَا حَاجَتُكُمْ إِلَى ذَلِكَ هَذَا أَبُو جَعْفَرٍ قَدْ أَجْلَسْتُهُ مَجْلِسِي وَصَيَّرْتُهُ مَكَانِي وَقَالَ إِنَّا أَهْلُ بَيْتٍ يَتَوَارَثُ أَصَاغِرُنَا عَنْ أَكَابِرِنَا الْقُذَّةَ بِالْقُذَّةِ.

2. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from Mu‘ammar ibn Khallad who has said the following. “I heard (abu al-Hassan) al-Rida who said something (leadership with Divine Authority) and then said, “I do not think you need what I just said. This is abu Ja‘far (a.s.). I have placed him in my own place to assume my position. We are of the family Ahl al-Bayt whose younger ones inherit from elder everything exactly measure to measure.”