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.

حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، حَدَّثَنَا ابْنُ شِهَابٍ، أَخْبَرَنِي خَارِجَةُ بْنُ زَيْدِ بْنِ ثَابِتٍ، أَنَّهُ سَمِعَ زَيْدَ بْنَ ثَابِتٍ ـ رضى الله عنه ـ يَقُولُ فَقَدْتُ آيَةً مِنَ الأَحْزَابِ حِينَ نَسَخْنَا الْمُصْحَفَ، كُنْتُ أَسْمَعُ رَسُولَ اللَّهِ ﷺ يَقْرَأُ بِهَا، فَالْتَمَسْنَاهَا فَوَجَدْنَاهَا مَعَ خُزَيْمَةَ بْنِ ثَابِتٍ الأَنْصَارِيِّ {مِنَ الْمُؤْمِنِينَ رِجَالٌ صَدَقُوا مَا عَاهَدُوا اللَّهَ عَلَيْهِ فَمِنْهُمْ مَنْ قَضَى نَحْبَهُ وَمِنْهُمْ مَنْ يَنْتَظِرُ } فَأَلْحَقْنَاهَا فِي سُورَتِهَا فِي الْمُصْحَفِ.