The seal of the Messenger of Allah ﷺ, meaning the one between his shoulders, was a red lump like a pigeon's egg.
حدثنا سعيد بن يعقوب الطالقاني حدثنا أيوب بن جابر عن سماك بن حرب
كان خاتم رسول الله ﷺ يعني الذي بين كتفيه غدة حمراء مثل بيضة الحمامة
No references are available.
Muslim also narrated it in Kitab al-Fadail, chapter on establishing the seal of prophethood and its description, etc., hadith no. 2344/109, from the hadith of Simak.