Praise be to Allah.

One of the features of the Prophet (peace and blessings of Allaah be upon him) that are mentioned in the ancient books is that between his shoulders was the Seal of Prophethood. That was a sign of his truthfulness, and that he was indeed the promised Prophet (peace and blessings of Allaah be upon him). 

The description of the Seal of Prophethood has been narrated in the saheeh Sunnah. It was an area of raised skin the size of a pigeon’s egg, between the shoulders of the Prophet (peace and blessings of Allaah be upon him). It was surrounded by moles and on it were some hairs. 

Al-Qurtubi said: 

The proven ahaadeeth are agreed that the Seal of Prophethood was something raised near his left shoulder, the size of a pigeon’s egg. 

There is no proof that the name of Allaah or the name Muhammad were written on the Seal, or any other words. 

Al-Haafiz ibn Hajar said in Fath al-Baari (6/650): 

With regard to what has been narrated about it being like the mark of a cupping glass, or like a black or green mole, or that the words “Muhammad Rasool Allaah (Muhammad the Messenger of Allaah)” or “Sir fa anta’l-mansoor (Go forth for you have Divine support)” etc were written on it, none of these reports have been proven. Do not be deceived by what was narrated in Saheeh Ibn Hibbaan because he was mistaken when he classed that as saheeh. And Allaah knows best. 

There follow some of the ahaadeeth that have been narrated concerning the Seal of Prophethood: 

1 – Muslim (2344) narrated that Jaabir ibn Samurah (may Allaah be pleased with him) said: “I saw the Seal by his shoulder, like a pigeon’s egg, resembling his body” i.e., its colour was like the rest of his body. 

2 – Muslim (2346) also narrated that ‘Abd-Allaah ibn Sarjis said: “I saw the Prophet (peace and blessings of Allaah be upon him), and I ate bread and meat with him (or he said, thareed – a meat dish)… then I went behind him and I looked at the Seal of Prophethood between his shoulders, near the top of his left shoulder, the shape of a hand with the fingers together (but smaller in size, the size of a pigeon’s egg), with moles on it.”  

See Sharh Muslim by al-Nawawi. 

3 – al-Tirmidhi narrated in al-Shamaa’il that Abu Zayd ‘Amr ibn Akhtab al-Ansaari said: “I touched his back, and my fingers touched the Seal.” He was asked, “What is the Seal?” He said, “A number of hairs growing together.” 

Classed as saheeh by al-Albaani in Mukhtasar al-Shamaa’il, p. 31 

And Allaah knows best.