The Seal of Muhammad (Turkish Muhr-e-Saadet or Muhr-e-Şarif; Arabic ختم الرسول) is one of the relics of Muhammad kept in the Topkapı Palace by the Ottoman Sultans as part of the Sacred Relics collection. It is allegedly the replica of a seal used by Muhammad on several letters sent to foreign dignitaries.
Jean-Baptiste Tavernier in 1675 reported that the seal was kept in a small ebony box in a niche cut in the wall by the foot of a divan in the relic room at Topkapi. The seal itself is encased in crystal, approximately 3"x4", with a border of ivory. It has been used as recently as the 17th century to stamp documents.
The seal is a rectangular piece of red agate, about 1 cm in length, inscribed with الله / محمد رسول (i.e. Allah "God" in the first line, and Muḥammad rasūl "Muhammad, messenger" in the second). According to Muslim historiographical tradition, Muhammad's original seal was inherited by Abu Bakr, Umar, and Uthman, but lost by Uthman in a well in Medina. Uthman is said to have made a replica of the seal, and this seal was supposedly found in the capture of Baghdad (1534) and brought to Istanbul.
A different design of the "seal of Muhammad" is circular, based on Ottoman era manuscript copies of the letters of Muhammad. This is the variant that has become familiar as the "seal of Muhammad" in jihadism, used in jihadist flags since ca. 2006, notably by the Islamic State of Iraq and the Levant/Daesh and by its sympathisers; some jewelry manufacturers have offered actual signet rings in this design long before the seal gained infamy through terrorist organisations.
The authenticity of the letters and of the seal is dubious and has been contested almost as soon as their discovery, although there is little research on the subject. Some scholars such as Nöldeke (1909) consider the currently preserved copy to be a forgery, and Öhrnberg (2007) considers the whole narrative concerning the letter to the Muqawqis to be "devoid of any historical value", and the seal to be fake on paleographical grounds, the writing style being anachronical and hinting at an Ottoman Turkish origin.