Sahih al-Bukhari 5670
Narrated As-Sa'ib:
My aunt took me to Allah's Messenger (ﷺ) and said, "O Allah's Messenger (ﷺ)! My nephew is- ill." The Prophet (ﷺ) 
touched my head with his hand and invoked Allah to bless me. He then performed ablution and I 
drank of the remaining water of his ablution and then stood behind his back and saw "Khatam An- 
Nubuwwa" (The Seal of Prophethood) between his shoulders like a button of a tent.
حَدَّثَنَا إِبْرَاهِيمُ بْنُ حَمْزَةَ، حَدَّثَنَا حَاتِمٌ ـ هُوَ ابْنُ إِسْمَاعِيلَ ـ عَنِ الْجُعَيْدِ، قَالَ سَمِعْتُ السَّائِبَ، 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 وَقُمْتُ خَلْفَ ظَهْرِهِ فَنَظَرْتُ إِلَى خَاتَمِ النُّبُوَّةِ بَيْنَ كَتِفَيْهِ مِثْلَ زِرِّ الْحَجَلَةِ‏.‏