Regarding Tarawih, it is recorded that one day, Umar Ibn Khattaab, visited the mosque in the second year of his caliphate during the last ten days of the holy month of Ramazan. (It should be borne in mind that the last ten days means the odd nights).
He sees the Muslims busy in worship. Some are in group of four, others in even more and yet another is all alone, all engrossed in worship. Umar thought to himself, ‘Every Muslim is worshipping separately. It would be wonderful if I make them all pray behind one Imam.’ With this intention, he ordered the people to pray these recommended prayers in a congregation and asked Ubayy Ibn Ka’b to lead it.
The people duly followed the instructions. No specific units were determined in this prayer because every Muslim was worshiping as per his ability, some more and others less. In not a single narration can one find the mention of the number of units. Yes, in order to prove their prayers as correct, every group regarded the innovation of Umar in units of eight, eleven, thirteen or twenty; and each one called the prayers of others as void.
But the reality is that its proof cannot be found in the words or actions of Umar because he himself never prayed the Tarawih prayers in his lifetime. He only ushered all the worshippers behind one Imam to gain reward for the same and visited the mosque everyday to supervise the same. Since this was a hard reality to digest, therefore only for the contentment of their hearts they justified that Umar used to pray his Tarawih prayers at home. Of course, there was no witness for the same.
Haafez Muhammad Abdullah Ghazipuri, a well-known Ahl Hadis scholar, after recording the narration of Ibn Abbas, comments, ‘From this explanation it can be deciphered that Umar himself did not observe the Tarawih congregational prayers. Perhaps, he believed that it is better to perform the same alone at home, especially in the last nights. Therefore, Imam Tahaawi Hanafi said that it is better to perform Tarawih prayers at home!!!”
Salafi cleric Karamuddin repeats the same words through Maulana Anwar Shah, “This approach is supported through the action of Umar that he used to pray the Tarawih prayers in the last nights in his house, although he had ordered the people to pray in congregation in the mosque!”
Anyways, the prayers had commenced although it was not given a name. People were praying only with the intention of recommended prayers. Days passed. This continued till the reign of the Ummayids. Every new caliph of the Bani Umayyah altered the units and timings of this prayer. Eight, thirteen, twenty, twenty-four, thirty-six and even forty units were prayed. Sometimes, it was prayed after the Eshaa prayers and at other times, late in the night.
It is narrated that some Muslims should take the support of a staff while praying this prayer due to the excessiveness of its units that they may not fall unconscious. When praying forty units consecutively became a grueling task, a law was made that some rest should be provided after every four units. This interval or rest is called as Tarveehah in Arabic.
In Tajalliyyaat-e-Safdar, Maulana Muhammad Ameen Safdar Saahab (Deobandi) explains the word Tarawih as follows: “This prayer was named Tarawih because people rested in it after every four units of prayers. For, Tarawih is the plural of Tarveehah which means to rest for some time. Tarawih is a plural noun and in Arabic, a plural is used for at least three or more than three. Hence, four units will be called as Tarveehah (singular), eight units will be called as Tarveehataan (dual) and twelve units will be called as Tarawih (plural). The Ummah is unanimous in this and this cannot be challenged with any Quranic verse or tradition.”
Since this law of rest or intermission came as an afterthought, hence the word Tarawih can neither be found in the Holy Quran nor any tradition.
 Rak’aat al-Tarawih, p. 39
 Tajalliyaat-e-Safdar, vol. 5, p. 29
excerpt from the book “Haqeeqat-e-Taraaveeh aur Bid’aat al-Sareeh’ by Ibn Haider