Hadithcore
Sahih al-Bukhari, 4787
sahih

Narrated Anas bin Malik:

The Verse: 'But you did hide in your mind that which Allah was about to make manifest.' (33:37) was revealed concerning Zainab bint Jahsh and Zaid bin Haritha.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ الرَّحِيمِ، حَدَّثَنَا مُعَلَّى بْنُ مَنْصُورٍ، عَنْ حَمَّادِ بْنِ زَيْدٍ، حَدَّثَنَا ثَابِتٌ، عَنْ أَنَسِ بْنِ مَالِكٍ ـ رضى الله عنه ـ أَنَّ هَذِهِ، الآيَةَ ‏{‏وَتُخْفِي فِي نَفْسِكَ مَا اللَّهُ مُبْدِيهِ‏}‏ نَزَلَتْ فِي شَأْنِ زَيْنَبَ ابْنَةِ جَحْشٍ وَزَيْدِ بْنِ حَارِثَةَ‏.‏

References2 variants
In-Book Reference
Book 65, Hadith 309
USC-MSA web (English) reference
Vol. 6, Book 60, Hadith 310 (deprecated numbering scheme)