كتاب الفضائل
43  
The Book of Virtues
(30)
Chapter: The Seal Of Prophethood, Its Attributes And Its Location On The Body Of The Prophet (SAW)
(30)
باب إِثْبَاتِ خَاتَمِ النُّبُوَّةِ وَصِفَتِهِ وَمَحِلِّهِ مِنْ جَسَدِهِ صلى الله عليه وسلم ‏.‏
Sahih Muslim 2345

As-Sa'ib b. Yazid reported:

My mother's sister took me to Allah's Messenger (ﷺ) and said: Allah's Messenger, here is the son of my sister and he is ailing. He touched my head and invoked blessings upon me. He then performed ablution and I drank the water left from his ablution; then I stood behind him and I saw the seal between his shoulders.
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، وَمُحَمَّدُ بْنُ عَبَّادٍ، قَالاَ حَدَّثَنَا حَاتِمٌ، - وَهُوَ ابْنُ إِسْمَاعِيلَ - عَنِ الْجَعْدِ بْنِ عَبْدِ الرَّحْمَنِ، قَالَ سَمِعْتُ السَّائِبَ بْنَ يَزِيدَ، يَقُولُ ذَهَبَتْ بِي خَالَتِي إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَتْ يَا رَسُولَ اللَّهِ إِنَّ ابْنَ أُخْتِي وَجِعٌ ‏.‏ فَمَسَحَ رَأْسِي وَدَعَا لِي بِالْبَرَكَةِ ثُمَّ تَوَضَّأَ فَشَرِبْتُ مِنْ وَضُوئِهِ ثُمَّ قُمْتُ خَلْفَ ظَهْرِهِ فَنَظَرْتُ إِلَى خَاتِمِهِ بَيْنَ كَتِفَيْهِ مِثْلَ زِرِّ الْحَجَلَةِ ‏.‏
Reference : Sahih Muslim 2345
In-book reference : Book 43, Hadith 145
USC-MSA web (English) reference : Book 30, Hadith 5792
  (deprecated numbering scheme)