Sahih al-Bukhari 23
Narrated Abu Said Al-Khudri:
Allah's Messenger (ﷺ) said, "While I was sleeping I saw (in a dream) some 
     people wearing shirts of which some were reaching up to the breasts 
     only while others were even shorter than that. Umar bin Al-Khattab was
     shown wearing a shirt that he was dragging." The people asked, "How 
     did you interpret it? (What is its interpretation) O Allah's Messenger (ﷺ)?"
     He (the Prophet (ﷺ) ) replied, "It is the Religion."
حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدِ اللَّهِ، قَالَ حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، عَنْ صَالِحٍ، عَنِ ابْنِ شِهَابٍ، عَنْ أَبِي أُمَامَةَ بْنِ سَهْلٍ، أَنَّهُ سَمِعَ أَبَا سَعِيدٍ الْخُدْرِيَّ، يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ بَيْنَا أَنَا نَائِمٌ رَأَيْتُ النَّاسَ يُعْرَضُونَ عَلَىَّ، وَعَلَيْهِمْ قُمُصٌ مِنْهَا مَا يَبْلُغُ الثُّدِيَّ، وَمِنْهَا مَا دُونَ ذَلِكَ، وَعُرِضَ عَلَىَّ عُمَرُ بْنُ الْخَطَّابِ وَعَلَيْهِ قَمِيصٌ يَجُرُّهُ ‏"‏‏.‏ قَالُوا فَمَا أَوَّلْتَ ذَلِكَ يَا رَسُولَ اللَّهِ قَالَ ‏"‏ الدِّينَ ‏"‏‏.‏