The seal of prophethood, as described in the biography of Prophet Muhammad, is a molar on his shoulder. This physical sign was given a special mention in the Old Testament and in some recently discovered codices.  These codices had been lost and proscribed by the successive synods.

The seal of the prophet-hood of prophet Muhammad was referred to some places but the Christian scholars consider it refereeing to something immaterial yet the Dead sea Scroll blows this claim as it gives a description of it as a physical sign.

 It is because of this sign( the prophethood molar) that many from the prophets contemporaries (Jews and Christians) believed in him. This is not the only evidence to his prophethood in the Holy Book.

 The biography of Prophet Muhammad tells us that many Jews and Christians believed in Muhammad the moment they saw this sign.

The description of the seal of prophethood from the Book of hadiths

In the highly authenticated  Hadiths, the companions of the prophet describe  that seal of prophethood  as follows:

It was a hairy lump of flesh on his left shoulder-blade ranging between a dove's egg and a small palm or fistful in size.  

Narrated As-Sa'ib:  My aunt took me to Allah's Apostle and said,

"O Allah's Apostle! My nephew is ill." The Prophet touched my head with his hand and invoked Allah to bless me. He then performed ablutionand 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. (Sahih Bukhari 7.574. see also Sahih Bukhari 4.741)

Another hadith explains that this is a large mole:

Narrated As-Sa'ib bin Yazid: My aunt took me to the Prophet and said,

"O Allah's Apostle! 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.) (Sahih Bukhari 1.189. see also Sahih Muslim 4.5793)