Allama Nawawi (rh) writes in his commentary of Muslim that the twenty rak’ah taraweeh is a hallmark of Islam just like the Eid salahs
(v.1 p.259). Therefore, the Ta’leeqaat of Hidayah (v.1 p.131) states that anyone performing only eight rak’ah of taraweeh will be guilty of forsaking the Sunnah.”
(1) Tarawih: Proof For 20 Rak'ats - http://www.muftisays.com/blog/Seifeddine-M/390_16-08-2010/tarawih.html
(2) The Number Of Rak'ats In Taraweeh - http://www.muftisays.com/blog/Seifeddine-M/614_01-12-2010/the-number-of-rak%27ats-in-taraweeh.html
(3) The Truth About The Number Of Rak'ats In Taraweeh - http://www.muftisays.com/blog/Seifeddine-M/586_20-11-2010/the-truth-about-the-number-of-rak%27ats-in-taraweeh.html
(4) Taraaweeh: 8 or 20? By Shaykh ul Hadeeth Riyadh Ul Haq - http://www.youtube.com/watch?v=mw99jkZboPA
(5) Taraweeh: A Reply To Dr Zakir Naik - http://www.muftisays.com/blog/abu+mohammed/392_23-08-2010/taraweeh.html
(6) Truth On The Tongue Of Hadhrat 'Umar (radiallaahu 'anh) - http://www.muftisays.com/blog/abu+mohammed/557_10-11-2010/truth-on-the-tongue-of-umar-%28r%29.html
(7) Following Hadhrat 'Umar (radiallaahu 'anh) - http://www.muftisays.com/blog/Seifeddine-M/1800_12-07-2011/following-hadhrat-%27umar-%28radiallaahu-%27anh%29.html
(8) Dr Zakir Naik vs Iqra Productions - http://www.youtube.com/watch?v=obnuz2Ht7s0&feature=channel_video_title
There is an ocean of evidence proving the legitimacy of 20 rak’ah taraweeh salah. The Sahabah (radiallaahu 'anhum) used to offer taraweeh in this manner. Not a single person from among the Sahabah (radiallaahu 'anhum) ever stepped forward to say that the number of rak’ah are not 20 but are 8.
Throughout the ages everyone agreed to the number being twenty, and now the ghair muqallideen all of a sudden claim that it is 8 rak’ah. They are going against practically all the Sahabah and the generations after them.
While the four Imams (Abu Hanifa, Shafi’, Malik, Ahmed Ibn Hanbal - may Allaah 'azza wa jal have mercy on them all) disagree in the total number of raka’ah of taraweeh, none of them has given a figure below 20. Taraweeh being 8 rak’ah is not supported by any of them.
The ‘ijma of the companions (radiallahu 'anhum) holds to be the last word on this subject, for the Prophet Muhammad (sallallaahu 'alaihi wa sallam) said that after him we should hold fast to the practices of the Khulafa ar-Rashideen.