Sahih Bukhari Volume 4, Book 52, Hadith Number 62.

Narated By Kharija bin Zaid : Zaid bin Thabit said, “When the Quran was compiled from various written manuscripts, one of the Verses of Surat Al-Ahzab was missing which I used to hear Allah’s Apostle reciting. I could not find it except with Khuzaima bin Thabjt Al-Ansari, whose witness Allah’s Apostle regarded as equal to the witness of two men. And the Verse was: “Among the believers are men who have been true to what they covenanted with Allah.” (33.23)