Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 379
Narrated Zaid bin Thabit:
When we wrote the Holy Quran, I missed one of the Verses of Surat-al-Ahzab which I used to hear Allah's Apostle reciting. Then we searched for it and found it with Khuzaima bin Thabit Al-Ansari. The Verse was:--
'Among the Believers are men Who have been true to Their Covenant with Allah, Of them, some have fulfilled Their obligations to Allah (i.e. they have been Killed in Allah's Cause), And some of them are (still) waiting" (33.23) So we wrote this in its place in the Quran.