Sahih Bukhari Volume 006, Book 060, Hadith Number 008.

Narrated By Ibn Abbas : Umar said, "Our best Qur'an reciter is Ubai and our best judge is 'Ali; and in spite of this, we leave some of the statements of Ubai because Ubai says, 'I do not leave anything that I have heard from Allah's Apostle while Allah: "Whatever verse (Revelations) do We abrogate or cause to be forgotten but We bring a better one or similar to it." (2.106)