It is mentioned in Al Fiqh Al Islami Wal Adilla under the Hanafi school of Fiqh:
Translation: Taraweeh is a Sunnah and there are 20 Raka’at of it, which are offered in units of 2 Raka’at. Witr will be offered after that. Apart from Ramadhan, Witr cannot be offered as a congregational prayer.
The practice of Hadhrat Umar (May Allah be well pleased with him) is the evidence of 20 Raka’at of Taraweeh as Imam Muslim (May Allah shower His mercy on him) has clarified this in his Sahih that Hadhrat Umar (May Allah be well pleased with him) gathered the Companions in the Masjd Nabvi to offer 20 Raka’at of Taraweeh and the Companions also agreed with him on this and even after the Caliphs, nobody objected to this.
(Al Fiqh Al Islami Wal Adilla, Al Juz Ath Thaani, Al Faslith Thamin, An Nawafil ‘Indal Hanafiyya)
Not only from the Companions, even the Successors (Taba’een) offered 20 Raka’at of Taraweeh, as we will see soon after.
Translation: Hadhrat ‘Abdullah bin Qais (May Allah be well pleased with him) narrates on the authority of Hadhrat Shaitar Bin Shakal (May Allah be well pleased with him) that he used to offer 20 Raka’at of Taraweeh and Witr in the month of Ramadhan.
(Sunan Kubra Lil Baihaqui, Hadith No: 4803; Musannaf Ibn Abi Shaiba, Vol 2, Pg No: 285, Bab #227, Hadith No: 1; Mirqatul Mafateeh Sharh Mishkatul Masabeeh, Kitab Us Salaat, Bab Qiyami Shahri Ramadhan)
Allama Ibn Hajr bin ‘Asqalani (May Allah shower His mercy on him) writes in Tahzeeb Ut Tahzeeb about Hadhrat Shaitar bin Shakal (May Allah be well pleased with him):
Translation: He is reliable and Imam Nasai (Author of Sunan Nasai, which is included in Sihah Sittah-The six most authentic books of Hadith) has also declared him as reliable. Ibn Hibban has written him among the reliable narrators. Ibn Sa‘ad has written that deductions can be made from him (i.e. his Hadith) and his narrations are less in number. ‘Ajli says that he is a reliable narrator and is a student of Hadhrat ‘Abdullah Bin Mas‘ud (May Allah be well pleased with him).
(At Tahzeeb Ut Tahzeeb, Vol 4, Pg No: 273/274)
Imam Ibn Hajr ‘Asqalani (May Allah shower His mercy on him) has written in Al Isaba Fi Ma’arifatis Companions:
Translation: Hadhrat Shaitar Bin Shakl ‘Abasi is a famous Taba‘ee. Abu Moosa Madini has written that he had the honor of been in the era of the Holy Prophet (Sallallahu alaihi wa sallam). (Al Isaba Fi Ma’arifatis Sahabah, Harfush Sheen)