The number of rak'ah is a long-standing and well-known point of difference. The practice traces to the Prophet Muhammad, who prayed the night prayer in Ramadan but, it is reported, declined to lead it regularly for fear it would be taken as obligatory. Under the caliph Umar the scattered worshippers were gathered behind a single leader, and the number that became customary in the great mosques of Mecca and Medina and across most of the Sunni world is twenty rak'ah. A significant body of scholars, citing reports of the Prophet's own night prayer, hold eight to be preferable, and both numbers are prayed today without either invalidating the other.
In Twelver Shia practice the additional Ramadan prayers are offered individually rather than in the congregational tarawih form, a difference rooted in differing views of its origin.
Umar's own description of his action, reported as ni'mat al-bid'ah hadhihi, what an excellent innovation this is, became a much-debated proof-text in later Islamic jurisprudence over whether any innovation (bidah) in religious practice can be praiseworthy, since strict traditionalists generally hold all religious bidah blameworthy by definition; scholars favoring the concept of a good innovation cite precisely this report about tarawih to argue that organizing an existing, individually practiced worship into a fixed communal form is a permissible and even commendable kind of change, distinct from introducing a genuinely new act of worship.