What was the number of rak’ahs in Tarawih prayer at the time of ‘Umar ibn al-Khattab (may Allah be pleased with him)?
Question

What is narrated from ‘Umar (may Allah be pleased with him) is that he ordered that Tarawih prayer should be twenty rak’ahs. Is that saheeh or da’eef? Or is there no basis for it?.
Praise be to Allah.Firstly: 
The report that ‘Umar ibn al-Khattaab ordered that Tarawih prayer should be twenty rak’ahs came from four of the Taabi’een. These are their reports: 
1 – It was narrated that Saa’ib ibn Yazeed said: ‘Umar ibn al-Khattaab (may Allah be pleased with him) gathered the people together in Ramadaan to be led by Ubayy ibn Ka’b and Tameem al-Daari in praying twenty-one rak’ahs, and they used to recite hundreds of verses, and they dispersed before dawn broke. 
A number of narrators narrated it from al-Saa’ib, some of whom mentioned twenty rak’ahs or twenty-one or twenty-three. They were: 
Muhammad ibn Yoosuf, the son of the sister of al-Saa’ib, from al-Saa’ib, as was narrated by ‘Abd al-Razzaaq in al-Musannaf (4/260) from the report of Dawood ibn Qays and others. 
Yazeed ibn Khusayfah. This was narrated by Ibn al-Majd in al-Musnad (1/413), and via him by al-Bayhaqi in al-Sunan (2/496). 
Al-Haarith ibn ‘Abd al-Rahmaan ibn Abi Dhubaab. This was narrated by ‘Abd al-Razzaaq in al-Musannaf (4/261). 
These reports are saheeh reports narrated by trustworthy narrators from al-Saa’ib ibn Yazeed. They mention twenty rak’ahs at the time of ‘Umar ibn al-Khattaab (may Allah be pleased with him). The extra amount mentioned – twenty-one or twenty-three, refers to Tarawih plus Witr. 
2 – It was narrated that Yazeed ibn Rumaan said: At the time of ‘Umar ibn al-Khattaab the people used to pray twenty-three rak’ahs of qiyaam in Ramadaan. 
This was narrated from him by Maalik in al-Muwatta’ (1/115). Al-Nawawi said in al-Majmoo’ (4/33: It is mursal, because Yazeed ibn Rumaan did not live at the same time as ‘Umar. End quote. 
3 – It was narrated from Yahya ibn Sa’eed al-Qattaan that ‘Umar ibn al-Khattaab (may Allah be pleased with him) ordered a man to lead them in twenty rak’ahs of prayer. This was narrated by Ibn Abi Shaybah in al-Musannaaf (2/163) from Wakee’ from Maalik. But Yahya ibn Sa’eed did not live at the same time as ‘Umar. 
4 – It was narrated that ‘Abd al-‘Azeez ibn Rafee’ said: Ubayy ibn Ka’b used to lead the people in praying twenty rak’ahs during Ramadaan in Madeenah, and he would pray Witr with three rak’ahs. 
This was narrated by Ibn Abi Shaybah in al-Musannaf (2/163). 
From all these reports it is clear that twenty rak’ahs was the way that Tarawih was usually done at the time of ‘Umar ibn al-Khattaab (may Allah be pleased with him). A matter such as Tarawih prayer is something that is well known among all people, and is transmitted from one generation to another. The report of Yazeed ibn Rumaan and Yahya al-Qattaan is to be taken into account even if they did not live at the same time as ‘Umar, because undoubtedly they learned it from a number of people who had been alive at the time of ‘Umar, and this is something that does not need and isnaad, rather all the people of Madeenah are its isnaad. 
Imam al-Tirmidhi (may Allah have mercy on him) said in his Sunan (3/169): 
Most of the scholars are of the view that what is narrated from ‘Umar, ‘Ali and other companions of the Prophet (peace and blessings of Allah be upon him) is twenty rak’ahs. This is the view of al-Thawri, Ibn al-Mubaarak and al-Shaafa’i. 
Al-Shaafa’i said: This is what I learned in our land, in Makkah they pray twenty rak’ahs. 
Ibn ‘Abd al-Barr said in al-Istidhkaar (2/69): 
Twenty rak’ahs was narrated from ‘Ali, Shateer ibn Shakl, Ibn Abi Mulaykah, al-Haarith al-Hamadaani and Abu’l-Bakhtari. It is the view of the majority of scholars and it is the view of the Kufis, the Shaafa’is and most of the fuqaha’. It was narrated in saheeh reports from Ubayy ibn Ka’b, and there was no difference of opinion among the Sahaabah. ‘Ata’ said: I grew up at a time when the people prayed twenty-three rak’ahs including Witr. 
See that in Musannaf Ibn Abi Shaybah (2/163). 
Ibn Taymiyah (may Allah have mercy on him) said in Majmoo’ al-Fataawa (23/112): 
It is proven that Ubayy ibn Ka’b used to lead the people in praying twenty rak’ahs in qiyaam in Ramadaan, and he used to pray Witr with three rak’ahs. Many scholars think that this is the Sunnah, because he established that among the Muhaajireen and Ansaar and no one objected to that. Others regarded it as mustahabb to pray thirty-nine rak’ahs, based on the fact that this was the practice of the people of Madeenah in the past. End quote. 
With regard to what it says in the report of Imam Maalik, Yahya al-Qattaan and others from Muhammad ibn Yoosuf from al-Saa’ib ibn Yazeed in al-Muwatta’ (1/115) and in Musannaf Ibn Abi Shaybah (2/162) “eleven rak’ahs” – this is to be understood as referring to what was done at first, then it was reduced after that, then ‘Umar increased the number to twenty to make the recitation in qiyaam easier for the people. 
Ibn ‘Abd al-Barr said in al-Istidhkaar (2/68): 
It may be understood as meaning that at first qiyaam at the time of ‘Umar was eleven rak’ahs, then he reduced the length of qiyaam for them and made it twenty-one rak’ahs, to make the recitation lighter for them and so that they would bow and prostrate more. But it seems most likely to me that the report about eleven rak’ahs is a mistake. And Allah knows best. End quote. 
Ibn Taymiyah (may Allah have mercy on him) said in Majmoo’ al-Fataawa (23/113): 
When Ubayy ibn Ka’b led them in praying qiyaam in a single congregation, he could not make them stand for too long, so he increased the number of rak’ahs to make up for the long standing. So they doubled the number of rak’ahs. He used to pray eleven or thirteen rak’ahs of qiyaam al-layl, then it seems that after that the people of Madeenah found it difficult to stand for so long during the recitation, so they increased the number of rak’ahs until it reached thirty-nine. End quote. 
Secondly: 
Night prayers are broad in scope, and there is no set number. Whoever wants to pray eleven rak’ahs may do so, and whoever wants to pray more or less than that may do so. The same applies to Tarawih prayers in Ramadaan. 
Ibn Taymiyah (may Allah have mercy on him) said in Majmoo’ al-Fataawa (23/113): 
One group said that it is proven in al-Saheeh from ‘Aa’ishah that the Prophet did not pray more than thirteen rak’ahs in Ramadaan or at any other time, and some people were uncertain about this report, because they thought that it contradicted the saheeh hadeeth and because of the practice of the Rightly-Guided Caliphs and the actions of the Muslims. 
But the correct view is that all of that is good, as was stated by Imam Ahmad (may Allah have mercy on him). There is no set number of rak’ahs for qiyaam during Ramadaan, because the Prophet (peace and blessings of Allah be upon him) did not set a number. End quote. 
Shaykh Ibn Baaz (may Allah have mercy on him) said in Majmoo’ al-Fataawa (11/322): 
It is proven that ‘Umar (may Allah be pleased with him) told the one whom he appointed among the Sahaabah to pray eleven rak’ahs, and it is proven that they prayed twenty-three rak’ahs based on his command. This indicates that the matter is broad in scope and that the matter was flexible according to the Sahaabah. That is also indicated by the words of the Prophet (peace and blessings of Allah be upon him): “The night prayers are two by two.” End quote. 
See also the answer to question no. 9036 and 38021. 
And Allah knows best.