Hadith 60-1

حَدَّثَنا الْحاكِم أَبُو عَلِي الحُسَيْنِ بْنِ أحْمَد الْبِيْهَقِيُّ قالَ حَدَّثَني مُحَمَّدِ بْنِ يَحْيَى الصُّوليُّ قالَ حَدَّثَنا عَوْنُ بْنِ مُحَمَّد قالَ حَدَّثَنا أَبُو الحُسَيْنِ بْنِ مُحَمَّدِ بْنِ أَبي عباد وَكانَ يَكْتُبُ لِلرِّضا عَلَيْهِ السَّلامُ ضمه إِلَيْهِ الفَضْلِ بْنِ سَهْل قالَ مَا كَانَ‏ عَلَيْهِ السَّلامُ يَذْكُرُ مُحَمَّداً ابْنَهُ‏ عَلَيْهِ السَّلامُ إِلا بِكُنْيَتِهِ يَقُولُ كَتَبَ إِلَيَّ أَبُو جَعْفَرٍ وَكُنْتُ أَكْتُبُ إِلَى أَبِي جَعْفَرٍ وَهُوَ صَبِيٌّ بِالْمَدِينَةِ فَيُخَاطِبُهُ بِالتَّعْظِيمِ وَتَرِدُ كُتُبُ أَبِي جَعْفَرٍ عَلَيْهِ السَّلامُ فِي نِهَايَةِ الْبَلاغَةِ وَالْحُسْنِ فَسَمِعْتُهُ يَقُولُ أَبُو جَعْفَرٍ وَصِيِّي وَخَلِيفَتِي فِي أَهْلِي مِنْ بَعْدِي.

Al-Hakim Abu Ali Al-Husayn ibn Ahmad al-Bayhaqi narrated that Muhammad ibn al-Sowli quoted on the authority of Oun ibn Muhammad, on the authority of Abul Husayn Muhammad ibn Abi Abbad - the one who was hired by Fadhl ibn Sahl to write for Imam Ar-Ridha’ (a.s.), “Ar-Ridha’ (a.s.) would never mention the name of his son Muhammad (a.s.) except by using his nickname (Abu Ja’far). For example, Ar-Ridha’ (a.s.) would say, ‘Abu Ja’far has written me a letter’ or say, ‘I wrote such and such to Abu Ja’far.’ This was so, even though his son was still a small child in Medina. However, the Imam (a.s.) always addressed him with respect. Abu Ja’far (a.s.) would also always respond in an extremely eloquent and charming manner. Once I heard Imam Ar-Ridha’ (a.s.) say, ‘Abu Ja’far (a.s.) is my Testamentary Trustee, and my successor from amongst my family after me.’”