Final answer:
Imam Ali al-Ridha is known as the 'Ornament of the Pious' among the Shia community, which recognizes the lineage of Imams descending from the Prophet Muhammad through his daughter Fatima and her husband Ali as the legitimate leaders of the Ummah.
Step-by-step explanation:
The Imam known as Ornament of the Pious is Imam Ali al-Ridha (c).
Shia Islam holds a unique belief in the leadership of the Ummah, considering only the descendants of Muhammad through his daughter Fatima and son-in-law Ali to be legitimate. Ali himself is respected as the first Imam in Shia tradition. Following him, a series of Twelve Imams are recognized, all of whom, including Ali al-Ridha, are considered by the Twelver Shia to have been divinely chosen to lead the Muslim community due to their direct descent from Muhammad.
In contrast, Sunnis respect Ali and the Twelve Imams but do not believe they were the sole divinely appointed leaders and think that any pious man following Muhammad's example could lead the Muslim community. The disagreement over leadership succession in Islam has historically defined the primary schism between Shia and Sunni Muslims.