ʿAlī 'ibn Mūsā ar-Reza(Arabic: علي إبن موسى الرضا) (commonly known as, ʿAlī ar-Riḍā) (ca. December 29, 765 -- August 23, 818) was the seventh descendant of the Apostol Muḥam·mad (ص) and the eighth of the Twelve 'A'im·mah of 'Imāmī, Shīʿī-'Islām.
Ali al-Ridha is considered an infallible Imam according to Shia Islam. Below are some historical notes about him.
The 8th Shi'ite Imam, Imam Ali ibne Musa ar-Reza (A.S.) is also called as Imam-e-Zaamin(A.S.).When Muslims go on journeys they take with them something called Imam-e-Zaamin (another name for Imam Reza (A.S.)) which is some money tied in a cloth. This tradition probably came from the time of Imam Reza (A.S.) when people used to carry money with them especially because it had Holy Imam's (A.S.) name on it. As Ma'mun,the Abbasid caliph the contemporary of Imam Reza(A.S.), had coins minted with the name of Imam Reza (A.S.) on them, it was very easy for the Muslims to carry the name of Holy Imam (A.S.) with them, especially when they went on journeys. Imam Reza (A.S.) is also known as Imam-e- Zaamin (A.S.). Zaamin means "The Guarantor". One day when Imam Reza (A.S.) was on his historical journey from Madina to Marw(Khurasan),while in a jungle the Holy Imam(A.S.) came across a hunter who was about to kill a deer. The deer was trying to get away and when she saw Holy Imam (A.S.), she said something to him.Holy Imam(A.S.) asked the hunter to free the deer so that she could go and feed her little baby deer who were very hungry. Imam Reza(A.S.) also told the hunter that once the deer had fed her babies she would come back. The hunter allowed the deer to go becauseHoly Imam (A.S.) had told him to, but he did not think the deer will come back. But Imam Reza (A.S.)) waited with the hunter until the deer returned with her young ones. The hunter was amazed on witnessing this miraculous event and he set the deer free as a mark of respect for Imam Reza(A.S.).After this historical event Imam Reza(A.S.) became famous as Imam Zaamin(A.S.). The most famous artist of the contemporary world, Ustad Mahmud Farshchiyan has depicted this historical event in this beautiful masterpiece iconograph entitled "Zaamin-e-Aahu"(The Guarantor of the Deer).