Hadithcore
Sahih al-Bukhari, 3234
sahih

Narrated Aisha:

Whoever claimed that (the Prophet) Muhammad saw his Lord, is committing a great fault, for he only saw Gabriel in his genuine shape in which he was created covering the whole horizon.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ إِسْمَاعِيلَ، حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ الأَنْصَارِيُّ، عَنِ ابْنِ عَوْنٍ، أَنْبَأَنَا الْقَاسِمُ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ مَنْ زَعَمَ أَنَّ مُحَمَّدًا رَأَى رَبَّهُ فَقَدْ أَعْظَمَ، وَلَكِنْ قَدْ رَأَى جِبْرِيلَ فِي صُورَتِهِ، وَخَلْقُهُ سَادٌّ مَا بَيْنَ الأُفُقِ‏.‏

References2 variants
In-Book Reference
Book 59, Hadith 45
USC-MSA web (English) reference
Vol. 4, Book 54, Hadith 457 (deprecated numbering scheme)