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 #4

4ـ عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ جَعْفَرِ بْنِ يَحْيَى عَنْ مَالِكِ بْنِ أَشْيَمَ عَنِ الْحُسَيْنِ بْنِ بَشَّارٍ قَالَ كَتَبَ ابْنُ قِيَامَا إِلَى أَبِي الْحَسَنِ (عَلَيْهِ السَّلام) كِتَاباً يَقُولُ فِيهِ كَيْفَ تَكُونُ إِمَاماً وَلَيْسَ لَكَ وَلَدٌ فَأَجَابَهُ أَبُو الْحَسَنِ الرِّضَا (عَلَيْهِ السَّلام) شِبْهَ الْمُغْضَبِ وَمَا عَلَّمَكَ أَنَّهُ لا يَكُونُ لِي وَلَدٌ وَالله لا تَمْضِي الايَّامُ وَاللَّيَالِي حَتَّى يَرْزُقَنِيَ الله وَلَداً ذَكَراً يَفْرُقُ بِهِ بَيْنَ الْحَقِّ وَالْبَاطِلِ.

4. A number of our people has narrated from Ahmad ibn Muhammad from Ja’far ibn Yahya from Malik ibn Ashyam from al-Husayn ibn Bashshar who has said the following. “Ibn Qiyaman wrote a letter to abu al-Hassan (a.s.) in which he had said the following. ‘How can you be an Imam when you do not have a son?” Abu al-Hassan al-Rida (a.s.) replied him with signs of anger, “How do you know that I will not have a son? By Allah, not many days and nights will pass before Allah will grant me a male child through who He will make the truth distinct from falsehood.”