Sahih al-Bukhari 4790
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · The Statement of Allah the Exalted: "...O you who believe! Enter not the Prophet's houses, except when leave is given to you for a meal... (up to) ... Verily! With Allah that shall be an enormity." (V.33:53)
Narrated `Umar:
حَدَّثَنَا مُسَدَّدٌ ، عَنْ يَحْيَى ، عَنْ حُمَيْدٍ ، عَنْ أَنَسٍ ، قَالَ قَالَ عُمَرُ ـ رضى الله عنه ـ قُلْتُ يَا رَسُولَ اللَّهِ، يَدْخُلُ عَلَيْكَ الْبَرُّ وَالْفَاجِرُ، فَلَوْ أَمَرْتَ أُمَّهَاتِ الْمُؤْمِنِينَ بِالْحِجَابِ، فَأَنْزَلَ اللَّهُ آيَةَ الْحِجَابِ.
I said, "O Allah's Messenger (ﷺ)! Good and bad persons enter upon you, so I suggest that you order the mothers of the Believers (i.e. your wives) to observe veils." Then Allah revealed the Verses of Al- Hijab.