20 Raka’at of Taraweeh - The command of Hadhrat Ali
There is a Hadith in Imam Baihaqui's Sunan Kubra and in ‘Umdatul Qari, Sharh Sahih Bukhari:
Hadhrat Abu Abdur Rahman Sulami narrates on the authority of Hadhrat Ali (May Allah be well pleased with him), that in the month of Ramadhan, he (i.e. Hadhrat Ali) summoned the Huffaz (those who memorized the Holy Quran) and commanded one Hafidh to lead others in 20 Raka’at of Taraweeh.
(Sunan Kubra Lil Baihaqui, Hadith No: 4804; Zujajatul Masabeeh, Vol 1, Pg No: 366; Umdatul Qari Sharh Sahih Bukhari, Kitab Ut Tahajjud; Tuhfatul Ahwazi, Bab Maa Ja'a Fi Qiyami Shahri Ramadhan)