Musnad Ahmad 938:68

Abu Kamil narrated to us, Ibrahim narrated to us, Ibn Shihab narrated to us, Kharijah ibn Zayd informed me that he heard Zayd ibn Thabit saying: "I missed an ayah from Surat al-Ahzab when we were transcribing the Mushafs. I used to hear the Messenger of Allah, may Allah bless him and grant him peace, reciting it: 'Among the believers are men true to what they promised Allah' [al-Ahzab: 23]. So I found it with Khuzaymah ibn Thabit, and I attached it to its surah in the Mushaf."