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" (ID)
English reference (ID): Book 2, Hadith 16 (ID)
Arabic reference (ID): Book 2, Hadith 17 (ID)