Saduq reported in his Uyun akhbarul Riza (1/157):

15 – حدثنا الحاكم أبو علي الحسين بن أحمد البيهقي قال: حدثني محمد بن يحيى الصولي قال: حدثنا أحمد بن القاسم بن إسماعيل قال: سمعت إبراهيم بن العباس يقول: لما عقد المأمون البيعة لعلي بن موسى الرضا عليهما السلام، قال له الرضا عليه السلام: يا أمير المؤمنين إن النصح لك واجب والغش لا ينبغي لمؤمن، أن العامة تكره ما فعلت بي والخاصه تكره ما فعلت بالفضل بن سهل والرأي لك أن تبعدنا عنك حتى يصلح لك أمرك قال إبراهيم: فكان والله قوله هذا السبب في الذي آل الامر إليه.

Al-Hakim Abu Ali Al-Hussein ibn Ahmad al-Bayhaqi narrated that Muhammad ibn Yahya al-Sowli quoted on the authority of Ahmad ibn Al-Qasim ibn Isma’il that he had heard Ibrahim ibn Al-Abbas say, “When Al-Ma’mun pledged allegiance to Ali ibn Musa Al-Reza (alaihi salam), the Imam (alaihi salam) told him, ‘O Commander of the Faithful! It is obligatory to be sincere with you. It is not proper for a believer to be tricky. The masses of the people do not like what you did to me. Especially, the elites do not like what you did with Al-Fadhl ibn Sahl. Now it is best for you to send both of us away from yourself until you can manage your own affairs.’” Ibrahim said, “By God! It was these words which resulted in what happened to him (death).”

And in same book you can read:

17 – حدثنا الحاكم أبو علي الحسين بن أحمد البيهقي قال حدثني محمد بن يحيى الصولي قال : حدثني أحمد بن محمد بن إسحاق قال : حدثنا أبي قال : لما بويع الرضا ع بالعهد اجتمع الناس إليه يهنئونه فأومى إليهم فأنصتوا ثم قال بعد أن استمع كلامهم : بسم الله الرحمن الرحيم الحمد لله الفعال لما يشاء لا معقب لحكمه ولا راد لقضائه (يعلم خائنة الأعين وما تخفي الصدور) وصلى الله على محمد في الأولين والآخرين وعلى آله الطيبين الطاهرين أقول وأنا علي بن موسى بن جعفر ع : إن أمير المؤمنين عضده الله بالسداد ووفقه للرشاد عرف من حقنا ما جهله غيره فوصل أرحاما قطعت وآمن نفوسا فزعت بل أحياها وقد تلفت وأغناها إذا افتقرت مبتغيا رضا رب العالمين لا يريد جزاء إلا من عنده (وسيجزي الله الشاكرين ولا يضيع أجر المحسنين ) وأنه جعل إلي عهده والإمرة الكبرى إن بقيت بعده فمن حل عقدة أمر الله تعالى بشدها وقصم عروة أحب الله إيثاقها فقد أباح حريمه وأحل محرمه إذا كان بذلك زاريا على الإمام منتهكا حرمة الإسلام بذلك جرى السالف فصبر منه على الفلتات ولم يعترض بعدها على الغرمات خوفا على شتات الدين واضطراب حبل المسلمين ولقرب أمر الجاهلية ورصد المنافقين فرصة تنتهز وبائقة تبتدر وما أدري ما يفعل بي ولا بكم ؟ أن الحكم إلا لله يقضي الحق

Al-Hakim Abu Ali Al-Hussein ibn Ahmad al-Bayhaqi narrated that Muhammad ibn Yahya al-Sowli quoted on the authority of Ahmad ibn Muhammad ibn Ishaq, on the authority of his father, “When they pledged allegiance to Al-Reza (alaihi salam), the people gathered around him and congratulated him. He made a gesture to them to become silent and after hearing their words, he said, ‘In the Name of God, the Beneficent, the Merciful. Praise be to God who does whatever He wills. No one can object to His Decrees. No one can reject His Destiny. He knows whatever treason that lies in one’s heart from the sight and whatever is hidden in the hearts. May God’s Blessings be upon Muhammad, upon the first and the last creatures, upon his good and pure Household.’ He then added, ‘I am Ali ibn Musa ibn Ja’far. Indeed the Commander of the Faithful (Al-Ma’mun) – may God assist him in righteousness and make him successful in being rational – has recognized one of our rights which others were ignorant about. He has established ties of kinship which others have broken. He has granted security to people who lived in fear. He even revived them and rescued them from destruction. He enriched them when they were in need. He was seeking the pleasure of the Lord of the Two Worlds in doing all this. He did not ask anyone but Him to reward him. God will reward the ones who are grateful. He will not let the reward of the good-doers be forgotten. He turned over the succession to the throne to me. He has entrusted his great rule to me after him. And whoever breaks a contract that God has ordered to be safeguarded, and loosens ties which God likes to be tightened has indeed underestimated God’s Bounds, and has allowed what God has forbidden. In this way, he has denied the leader of his right and has disobeyed his orders. He has thus disrespected Islam as was done in the past. The Trustee (Ali ibn Abi Talib (alaihi salam)) put up with all the instances of breaching of the covenants, and did not even object to those later when he was in power, fearing dispersion in the religion and turmoil in the ties which held the Muslims together. This was because the ideas of the Age of Ignorance were still in their minds and the hypocrites were on the watch to get a chance to create havoc. Now I do not know what is going to happen to me and you. Indeed there is no arbitrator but God. He is the only One who clarifies the truth. He is the best Separator.”