Ash-Shama'il Al-Muhammadiyah, Book 2, Hadith 16 (English) ⓘ

Jaabir bin Samurah (Radhiallahu Anhu reports that: "I saw the Seal of Prophethood of Rasullullah (Sallallahu alaihi wasallam) between his two shoulders, which was like a red tumor (protruding flesh), the size of which was like that of a pigeon's egg"

حَدَّثَنَا سَعِيدُ بْنُ يَعْقُوبَ الطَّالْقَانِيُّ، قَالَ‏:‏ حدَّثَنَا أَيُّوبُ بْنُ جَابِرٍ، عَنْ سِمَاكِ بْنِ حَرْبٍ، عَنْ جَابِرِ بْنِ سَمُرَةَ، قَالَ‏:‏ رَأَيْتُ الْخَاتَمَ بَيْنَ كَتِفَيْ رَسُولِ اللهِ صلى الله عليه وسلم، غُدَّةً حَمْرَاءَ، مِثْلَ بَيْضَةِ الْحَمَامَةِ‏.‏