Musnad Ahmad 938:65

Al-Hakam ibn Nafi' narrated to us, Shu'ayb informed us, from al-Zuhri, Kharijah ibn Zayd informed me, that Zayd ibn Thabit said: When we transcribed the copies of the Qur'an, I missed an ayah from Surah al-Ahzab that I used to hear the Prophet, peace and blessings of Allah be upon him, reciting. So I searched for it and did not find it with anyone except with Khuzaymah ibn Thabit al-Ansari, whose testimony the Messenger of Allah, peace and blessings of Allah be upon him, made equal to the testimony of two men. It is the saying of Allah, the Almighty and Majestic: "Among the believers are men true to what they covenanted Allah for" [al-Ahzab: 23].