Sahih al-Bukhari, Hadith 190 ⓘ

Narrated As-Sa'ib bin Yazid: My aunt took me to the Prophet and said, "O Allah's Messenger! This son of my sister has got a disease in his legs." So he passed his hands on my head and prayed for Allah's blessings for me; then he performed ablution and I drank from the remaining water. I stood behind him and saw the seal of Prophethood between his shoulders, and it was like the "Zir-al-Hijla" (means the button of a small tent, but some said 'egg of a partridge.' etc.)

حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ يُونُسَ، قَالَ حَدَّثَنَا حَاتِمُ بْنُ إِسْمَاعِيلَ، عَنِ الْجَعْدِ، قَالَ سَمِعْتُ السَّائِبَ بْنَ يَزِيدَ، يَقُولُ ذَهَبَتْ بِي خَالَتِي إِلَى النَّبِيِّ 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 خَاتَمِ النُّبُوَّةِ بَيْنَ كَتِفَيْهِ مِثْلِ زِرِّ الْحَجَلَةِ‏.‏