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'A'isha said, "I used to eat date and butter mixture with the Prophet, may Allah bless him and grant him peace. 'Umar visited and the Prophet invited him and he ate. 'Umar's hand touched my fingers and he said, 'Oh! If I were to be obeyed about you, no eye would ever see you!' Then the veil was revealed."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا الْحُمَيْدِيُّ، قَالَ: حَدَّثَنَا سُفْيَانُ، عَنْ مِسْعَرٍ، عَنْ مُوسَى بْنِ أَبِي كَثِيرٍ، عَنْ مُجَاهِدٍ، عَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: كُنْتُ آكُلُ مَعَ النَّبِيِّ صلى الله عليه وسلم حَيْسًا، فَمَرَّ عُمَرُ، فَدَعَاهُ فَأَكَلَ، فَأَصَابَتْ يَدُهُ إِصْبَعِي، فَقَالَ: حَسِّ، لَوْ أُطَاعُ فَيَكُنَّ مَا رَأَتْكُنَّ عَيْنٌ. فَنَزَلَ الْحِجَابُ.
sunnah.com