During the lifetime of Imam Reza (peace be upon him), the Caliph of the time, Mamun, requested that he briefly write about the pure essence of Islam. In response, the Imam wrote a significant text, which can be summarized as follows:
“Pure Islam is the testimony of the oneness of the deity, that there is no god but Him. He is without comparison. He is one and unique, without likeness or resemblance. He is self-sufficient and eternal. He has no beginning and is self-sustaining. He is hearing, seeing, and all-powerful. He is eternal and everlasting. He is knowledgeable, and nothing is hidden from Him. He is capable, and weakness does not affect Him. He is self-sufficient, and nothing can make Him in need. He is just, and no oppression comes from Him. He is the Creator of everything. He has no equal, partner, or resemblance. He is the only one worthy of worship and supplication.
And the one whom Muhammad is, the servant, messenger, trustworthy, chosen and elected from among His creation, the master and leader of the prophets, the seal of the divine prophets, and after him, there will be no other prophet. His religion will not be altered, and all that Muhammad bin Abdullah brought is clear truth. We affirm the truth of his book, which is sacred and untainted by falsehood. This book (the Quran) contains the truth from beginning to end, from the opening to the conclusion, and we believe in its verses, which no creation can replicate.
And the one who is the guide of humanity after him (Prophet Muhammad), the guardian of the believers, the Imam of the Muslims, the interpreter of the Quran and its laws, is his brother and successor, the one responsible for his family. He is Ali ibn Abi Talib, the Commander of the Faithful (peace be upon him), the Imam of the pious, the best of the successors, and the inheritor of all the knowledge of the prophets. After him, Hasan and Husayn, the masters of the youth of Paradise, followed by Ali ibn Husayn, Zain al-Abidin, then Muhammad ibn Ali, the one who explained the difficult issues of the prophets’ sciences, followed by Ja’far ibn Muhammad al-Sadiq, the heir of the knowledge of the successors, then Musa ibn Ja’far al-Kadhim, who subdued anger, then Ali ibn Musa, after whom comes Muhammad ibn Ali, followed by Ali ibn Muhammad, then Hasan ibn Ali, and finally, the awaited Mahdi, the Promised Savior, upon whom be God’s peace and blessings.
These are all successors, Imams, and leaders of the religion. The earth will never be devoid of the proof of God, and people will never be without a divine guide in any age or time.”[1][2]
(Adapted from the book Uyun Akhbar al-Reza, compiled by Shaykh Saduq, with some summarization)[3]
Footnotes:
[1] Uyun Akhbar al-Reza, Volume 2, page 121
[2] Imam Reza (peace be upon him), in the continuation of his noble letter, after introducing the Imams following the Prophet Muhammad (peace be upon him), goes on to mention some divine laws that the Islamic community is faced with. These can be referred to in the Uyun for further understanding.
[3] The above text is adapted from the Persian translation of Uyun Akhbar al-Reza by Hamid Reza Mostafid and Ali Akbar Ghafari.