Reading behind Imam

Rasulullah (Sallallaahu Alaihi Wasallam) said: 

"The one who has an Imam, then the Qiraat of the Imam is absolutely his Qiraat."

This Hadith shows that one should not read behind the Imam, because what the Imam recites is valid on behalf of those who are following him. 

Regarding this Hadith, Allaamah Ibn Taimiyyah said that it is a Hadith that the majority of the Sahaabah-e-Kiraam (Radiyallahu Anhum) and the Taabi'een have practiced. And such a Hadith is a Hujjah (proof) according to the unanimity of all 4 Imam. (Fataawaa Bin Taimiyyah)

[English version of "Lire derrière Imam" – Al Jamiat – Vol.2 No.6]