Answer
In the name of Allah, the Most Compassionate, the Most Merciful.
All praise belongs to Allah. The shrine in Cairo that people attribute to Ḥusayn ibn ʿAlī (peace be upon him) is nothing but a fabricated myth. On this point there is no disagreement among the well-known scholars to whom Muslims turn in such matters because of their knowledge and trustworthiness. Not a single named authority, recognized for both learning and honesty, has ever declared the shrine to be authentic. The claim is repeated only by certain individuals who cite unknown sources, following the practice of those who retail the assertions of the Rāfiḍites (1) and other habitual liars. They transmit reports and anecdotes and put forward doctrines and opinions, and when
(1) Among the Sab'iyyah Rafidah—those who displayed their innovation during the time of Ali (may Allah be pleased with him)—there were some who said, ‘Perhaps you are the deity.’ They consequently burned some of their people and exiled their leader, Abdullah ibn Saba, to Sabat al-Mada'in. This does not constitute an Islamic sect, since they declared that Ali is a deity.
After the time of Ali, however, the Rafidah split into four groups: the Zaydi and the Imamiyyah, the Kaysaniyyah, and the Ghulat. The Zaydis, the Imamiyyah, and the Ghulat each fragmented into separate sects, and each sect declared the others to be unbelievers. All of the Ghulat groups are outside the bounds of Islam. Al-Baghdadi considered the Zaydi faction to be one of the Rafidah, even though the Zaydis follow Zayd, unlike the others who supported him and later abandoned him.
For further details, see Differences Among the Sects by al-Baghdadi (in the edition by Sheikh Muhammad Mahyiddin Abdullah al-Hamid, p. 21, abbreviated), Dar al-Ma'arifa (Lebanon), Maqalat al-Islamiyyin (1/129), and Muruj al-Dhahab by al-Mas'udi (3/220). Also, for the Kaysaniyyah, refer to Muruj al-Dhahab (3/87); and regarding the Imamate, al-Mas'udi discusses it as well (3/236).