Hafiz Ibn Taymiyyah writes:

It has been proved that Ubay bin Ka‘b used to offer twenty rak‘ahs with the people in the Qiyam of Ramadan. Adding to them, three more. Many of the scholars have said that this is the sunnah, as the emigrants and the Madinan helpers used to stand with him, and they did not object to it.

(Majmu‘ al-Fatawa by Hafiz Ibn Taymiyyah, under Naza‘ al-Ulama’ fi miqdar qiyam Ramadan)

Hafiz Ibn Taymiyyah (RA) writes in his Fatawa: ‘Verily it has been proven that Umar made Ubay ibn Ka’ab (RA) lead the people in 20 Rakats of Taraweeh and 3 Witr in Ramadhan. This is the way of the scholars of this Ummah and this is the Sunnah. This is due to the fact that Sayyidina Ubay ibn Ka’ab (RA) read 20 Rakats Taraweeh in a large congregation of the Muhajireen and Ansar of the Sahabah and none amongst them objected to this practice.’ (Fatawa Ibn Taymiyyah vol 1, p191/ Al Mughni vol 1, p803)

In the time of Hafiz Ibn Taymiyyah (RA) when the Rawafid (Shia) put the blame upon Sayyidina Umar (RA) for creating the innovated practice of 20 Rakats Taraweeh. Ibn Taymiyyah (RA) responded to this allegation by writing in defence of Sayyidina Umar (RA) to the Rawafid: ‘If Sayyidina Umar Farooq (RA) by establishing 20 Rakats of Taraweeh had been deemed to adopt a bad practise, then Sayyidina Ali (RA) would have put an end to this in his Khilafat. However, in the Khilafat of Sayyidina Ali (RA) , he too would read 20 Rakats Taraweeh prayer in Kufa. In the month of Ramadhan, Sayyidina Ali (RA) would state: ‘May Allah enlighten the grave of Sayyidina Umar (RA) , just as He (Umar ) has enlightened our Masajids for us.’ (Due to the fact of establishing the 20 Rakats Taraweeh for the Ummah). (Minhaaj ul Sunnah vol 2, p224)