Sahih al-Bukhari 4783
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · "Of them, some have fulfilled their obligations (i.e ., have been martyred) and some of them are still waiting, but they have never changed (i.e., they never proved treacherous to their covenant which they concluded with Allah) in the least." (V.33:23)
Narrated Anas:
حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ ، حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ الأَنْصَارِيُّ ، قَالَ حَدَّثَنِي أَبِي ، عَنْ ثُمَامَةَ ، عَنْ أَنَسِ بْنِ مَالِكٍ ـ رضى الله عنه ـ قَالَ نُرَى هَذِهِ الآيَةَ نَزَلَتْ فِي أَنَسِ بْنِ النَّضْرِ {مِنَ الْمُؤْمِنِينَ رِجَالٌ صَدَقُوا مَا عَاهَدُوا اللَّهَ عَلَيْهِ} .
We think that the Verse: 'Among the Believers are men who have been true to their covenant with Allah.' was revealed in favor of Anas bin An-Nadir.